Imaan (Faith) & Islam (Self-Submission)
The importance of Al-ghayb (unseen) is very essential part of a Muslim’s Imaan (faith). Without belief in it, one cannot have any faith in Islam. Al-ghayb includes everything that is hidden from our normal senses and perceptions, which includes Allah ﷻ, His angels, jinns (invisible beings), judgment day, existence of hell and paradise, the souls of the dead, life of barzakh (intermediary stage between death and resurrection) and that of hereafter. Prophet Muhammad ﷺ taught that to have faith one has to believe in the following: “Imaan (faith) is to believe in Allah, His Angels, His Books, His Prophets, and the meeting with Him, and to believe in the Resurrection.” All these things belong to the unseen. Al-ghayb does not only include what remains hidden from us in the future but also includes what happened in the past. All the events related to previous prophets are included within what is described as having faith in “His Books, His Prophet”’. The necessity of having faith in all such unseen matters is stressed right in the beginning of the Quran: (Al Quran 2:03) Who have Imaan (faith) in the al-ghayb (unseen), establish salah (prayer), and spend out of what We have provided for them.” The mention of al-ghayb precedes that of salaah (prayer) and zakat (charity) because all righteous actions should be based on belief in al-ghayb. Since it is essential part of Islamic faith, any actions done without this essential factor is equivalent to having no faith at all and Quran teaches us this: (Al Quran 6:158) Are they waiting for the very angels to appear, or your Lord Himself, or maybe some of His signs (such as Day of Judgment)? But on the day some of your Lord’s signs appear, faith will be of no avail to any human being who did not believe before.” Allah ﷻ and His angels are hidden from us in the unseen and will appear on judgment day, thus, we are informed that to believe in Him and His angels is part of faith and absolute essential for doing righteous actions. Any doubts in the concept of al-ghayb will be equivalent to having no faith at all even if one does many good works. There are two angels assigned over each human recording all our actions, as Allah ﷻ said: (Al Quran 50:17) There are two scribes, one each sitting on the right and the left, recording everything.” This record will be provided to us on the Day of Judgment, which is also a major event that lies in the unseen in future: (Al Quran 36:12) and We shall record whatever [deeds] they have sent ahead, and the traces [of good and evil] which they have left behind: for of all things do We take account in a record clear.”
Al-ghayb also includes satan and his group of jinn devils who tempt us to evil. To protect ourselves from their machinations it is must to have faith in this matter: (Al Quran 7:27) let not the satan cause you to fall into affliction ….he surely sees you, he as well as his group, from where you cannot see them.” This knowledge pertaining to satan and his tricks is a matter of unseen which is utmost necessary to believe, in order to protect ourselves. Since we were created to know, worship and be obedient to Allah ﷻ, therefore, whatever we do in this life we will experience the results of it in the hereafter, a major event in unseen. Paradise in Arabic is called Jannah, meaning something that is “hidden”, covered from our normal perceptions, is also part of al-ghayb. Allah ﷻ promises us: (Al Quran 32:17) no soul knows what is hidden for them of that which will refresh the eyes; a reward for what they did. The best reward that will refresh the eyes of those who had faith in al-ghayb and lived in accordance to it, is the sight of their Lord which is impossible to happen in this world as He said: (Al Quran 6:103) No human vision can encompass Him, whereas He encompasses all human vision: for He alone is Unfathomable, AllAware. The promise of vision can only be fulfilled in the hereafter for those who enter paradise: (Al Quran 75:22–23) [Some] faces, that Day, will be radiant, Looking at their Lord. Indeed what could be best pertaining to the importance of this concept except the sight of our Creator and Lord. Thus faith in al-ghayb is utmost necessary in Islam without which we are unable to understand and fulfill purpose of our life on earth.
 Sahih Bukhari Vol. 6, Book 60, Hadith 300
 Al Quran 2:03 – Asad Translation.
 Al Quran 6:158
 Al Quran 50:17
 Al Quran 36:12 – Asad Translation.
 Al Quran 7:27
 Al Quran 32:17
 Al Quran 6:103 – Asad Translation.
 Al Quran 75:22–23 – Asad Translation.
Becoming a Muslim does make you a Mumin (believer) however there are certain guidelines that a Muslim need to adhere to so that their faith does not decrease. In simple terms they need to live with care so that they are not just Muslims by name. Some Bedouins came to the Prophet ﷺ to accept Islam and proudly declared that they now have faith, to which the following verse of the Quran was revealed: (Al Quran 49:14) THE BEDOUIN say, ‘We have attained to faith.’ Say [to them, O Muhammad]: ‘You have not [yet] attained to faith; you should [rather] say, 'We have [outwardly] surrendered' - for [true] faith has not yet entered your hearts. But if you [truly] pay heed to God and His Messenger, He will not let the least of your deeds go to waste: for, behold, God is much-forgiving, a dispenser of grace.” A Muslim is the one who submits his will to the will of Allah i.e. accepts Islam and Mumin is the one who has attained to faith as a result of that submission. In order to become a Mumin one has to first become a Muslim. In order to understand the difference between Muslim and Mumin we need to understand importance of Islam (self-submission) and Imaan (faith).
Islam (self-submission) and Imaan (faith) complement each other. As a sheikh once said that, it is a circle within a circle. The circle of Imaan lies within the circle of Islam. Therefore, one has to enter the circle of Islam first in order to enter that of Imaan. It also holds the position of a heart within a body. The body symbolizes as Islam but the heart inside the body is symbolized as Imaan. If there was no heart the body would be lifeless just as without Imaan Islam would be meaningless. A prophetic hadith teaches that: “there is a piece of flesh inside the body which if it is good then the whole body is good, but if it is corrupt then the whole body will suffer. That piece is the heart.” The functioning of the heart plays a primary role in sustaining the life of our body and so does Imaan in sustaining one’s Islam. If Imaan is strong then our deen (way of life) will be strong and if that is weak then it will be difficult to remain steadfast in Islam.
The emotional attachment towards anything personal in one’s life is dependent upon it being closer to one’s heart. The Imaan begins to enter the heart once we decide to submit our will to the will of Allah i.e. accept Islam and begin following His guidance. The circle of Islam is to give shahadah i.e. testify that la ilaha ilAllah Muhammadur rasool Allah (There is no god but Allah and Muhammad is His Messenger of Allah), to pay zakat (obligatory charity), pray five times salah (prayers), fast the month (thirty days) of ramadaan and perform hajj and umrah if one has the financial ability. These are called five pillars of Islam. In order to taste the sweetness of Imaan an individual has to accept the five pillars and follow them sincerely. The following incident explains how the journey of a Muslim begins with acceptance of Islam with Imaan eventually entering the heart.
A man came to the Prophet ﷺ and chose to accept Islam on the condition that he should be allowed to pray only two prayers instead of compulsory five, to which the Prophet ﷺ allowed him. He accepted Islam by verbal shahadah, became a Muslim and left. Amazed the companions inquired the reason for allowing him to shorten the number of prayers to which the Prophet ﷺ replied “let him begin with two, once he tastes the sweetness of Imaan (faith) he will want to do five.” The verses of Al Quran 49:14 were revealed as a response to arrogant manner of the bedouins, to correct them that they need to first humble themselves and make the intention of obeying the rules and regulations of Islam before Imaan begins to gradually enter their hearts. The obedience is a condition for faith, as said in the above 49:14 “If you obey God and His messenger, He will not put any of your works to waste.” Rules and regulations are given by Allah ﷻ and His Messenger ﷺ and so they needed to obey them first. There is a saying of one of the earlier jurists of Islam named Abu Ubayd Al Qasim ibn Salaam who when inquired about Imaan said “Imaan is to have sincere devotion to God with the heart, to testify this with the tongue, and to act on it with the body.”
Therefore, a man may verbally accept Islam and become a Muslim but in order to become a Mumin he has to become a Muslim who wholeheartedly obeys Allah ﷻ and His Messenger ﷺ and practices Islam with sincerity. One very important thing to mention here is that we Muslims cannot judge any one’s faith, as it is an internal matter within one’s heart. We can only identify a Muslim through their outward actions and not their Imaan, as Imaan is something internal and hidden and the knowledge of unseen only rests with Allah ﷻ.
For more info on what is Imaan and Islam please read the above topic: What is Imaan and Islam?
 Al Quran 49:14 – Asad Translation.
 Sahih Muslim 1599 a
 ‘There is no god but Allah’ is called shahadah (declaration of faith) that Muslims often utter as a sort of zikr (reminder) in their prayers. This declaration is first of the five pillars of Islam.
 770–838 CE
 Heard in one of the khutbah (lectures) in masjid.
The importance to ascertain a persons legal status as per Islamic Law.
First we need to understand why is it important to ascertain the legal status of a person as per Islamic law before answering if a Muslim believer can be a Kafir (disbeliever) or no. Leading scholar Sheikh Hamza Yusuf of Zaytuna Institute, says the legal status of a person is important to ascertain because of consequences of such legal standing with regard to marriage, birth, death, property inheritance etc. A Kafir does not inherit from Muslims, nor do Muslims inherit from a kafir. Moreover, a kafir is not buried according to Islamic funeral rites, nor is he or she prayed for after death by Muslims. A kafir cannot marry a Muslim neither can a Muslim marry a kafir.
Difference between a Mumin (Believer) vs a Kafir as per Islamic law
Since we now know the importance of the legal status of person in an Islamic society and culture and how it affects the society, we now need to know the difference between a Mumin and a Kafir. There can be a long discussion on who is a kafir in the light of Quran and much has been written about it by Islamic scholars throughout the Islamic history. But to keep it short and simple, Kufr (disbelief) is opposite of imaan (faith) and a Kafir (disbeliever) is opposite of Mumin (a believer). As per Islamic law anyone who believes in Allah ﷻ as the creator and accepts the message brought by Prophet Muhammad ﷺ is considered to be a Mumin and anyone who disbelieves in Allah ﷻ as their creator and rejects the message of Prophet Muhammad ﷺ is considered to be outside of Islam and hence a disbeliever. This is a simple definition of a Kafir vs a Mumin (believer) without going in to hefty details of these terms.
What are essentials of accepting Imaan (Faith) and Islam?
In the light of Islamic law, as long as someone accepts the following articles of Imaan (Faith) and Islam, he or she can not be taken outside the fold of Islam and called a Kafir. Messenger ﷺ said, "Faith is to believe in Allah, His Angels, His Books, His Messengers, and the meeting with Him, and to believe in the Resurrection." In another narration, he also added “…and that you believe in qadr (divine decree or predestination) in its entire entirety.” The Quran also confirms these articles as part of Islamic faith: (Al Quran 2:285) The Messenger has believed in what was revealed to him from his Lord, and [so have] the believers. All of them have believed in Allah and His angels and His books and His messengers, [saying], "We make no distinction between any of His messengers.” The Prophet ﷺ also said “al-Islam is raised on five (pillars), testifying (the fact) that there is no god but Allah, that Muhammad is His servant and messenger, and the establishment of salah (5 times prayer), payment of zakat (obligatory charity), pilgrimage (hajj & umrah) to the house (kaabah) and the fast of ramadan.”
So as long as someone believes in the above 6 or 7 articles of faith & 5 pillars of Islam, cannot be called a Kafir (disbeliever).
Actions that constitute as Kufr (disbelief) but does not turn Muslim Believer in to Kafir
In the light of Islamic law, a Muslim may commit acts of Kufr (disbelief) but he cannot be declared a Kafir (disbeliever), meaning that such acts do not take him or her outside of the fold of Islam. Such as a prophetic hadith teaches that a Muslim believer is not considered in his or her imaan only at the time of committing sinful actions: “The adulterer is not a believer at the moment when he (or she) is committing adultery, and the wine drinker is not a believer at the moment when he is drinking wine, and the thief is not a believer at the moment when he is stealing, and the robber is not a believer at the moment when he is robbing and people are looking on." 
This hadith teaches that a believer may commit acts of kufr (disbelief) but remains within the folds of Islam and Imaan (faith), except that their Imaan is considered weak at the time of their committing such actions.
How a Muslim can be declared a Kafir (disbeliever) and the importance of this legal status.
Legally, as per Islamic law, there are certain conditions taken in to consideration before qaazi (Islamic judges) can declare a Muslim to be a Mumin or a Kafir. To ascertain this status the Islamic jurists agreed upon guidelines and six conditions have to be fulfilled before a judge can rule concerning a person’s faith: Proof of Faith; Intention; Absence of Coercion; Level of Knowledge; Absence of Esoteric Interpretation; and Mental Ability to Reason.
These six conditions need to be fulfilled before a Muslim believer can be declared a Kafir by the experts of Islamic law and not otherwise.
 Who are the Disbelievers by Shaykh Hamza Yusuf, Zaytuna Institute, Seasons 200 - Spring 2008 - https://zaytuna.edu/WhoAretheDisbelievers.pdf
 Sahih Bukhari - Book 2, Hadith 43 - Sunnah.com
 Sahih Muslim – Hadith 10 - Sunnah.com
 Sahih Muslim Hadith 16 - Sunnah.com
 Sunan an-Nasa'i 5659 - Sunnah.com
 Musnad Ahmad 940, 1361, Sunan an-Nasai 3432 - Sunnah.com
(Al Quran 3:108) Allah does not will injustice for His creatures
Allah ﷻ does not will to punish people, in fact it is their own actions that will result in their punishment. Allah ﷻ only punishes those who knowingly and willingly reject His Message, He will not punish those who either didn’t get to hear His message or did hear but were unable to comprehend due to whatever valid reasons that He is aware of, as He is Truly All Knower and All Seer of each individual's life, as implied in the following verse: (Al Quran 6:131–132) And so it is that Your Sustainer would never destroy a community for its wrongdoing so long as its people are still unaware [of the meaning of right and wrong]: for all shall be judged according to their [conscious] deeds - and Your Sustainer is not unaware of what they do.
A very important lesson can also be derived from one of the most recited Surah of the entire Quran, Al Fatiha (The Opening), the last verse of which some of greatest thinkers of Islam have interpreted as identifying such people who never got the message of Islam or after receiving were unable to comprehend due to valid reasons: (Al Quran 1:07) the way of those upon whom You have bestowed Your blessings, not of those who have been condemned [by You], nor of those who go astray! - In his Message of the Quran - Muhammad Asad shares the views of Imam Ghazali (r.a) (9th Century Islamic Thinker) who held that the people described as "those who have been condemned" - that is, having deprived themselves of God's grace - are those who have become fully cognizant of God's message and, having understood it, have rejected it; while "those who go astray" are meant people whom the truth has either not reached at all, or to whom it has come in so garbled and corrupted form as to make it difficult for them to recognize it as the truth.
The above explanation of this verse indicates that there are people that Quran mentions who are not aware of the truth and they are ignorant therefore they will be judged accordingly and Allah ﷻ is not unaware of their conditions. It is this reason why He separated "those who go astray" in this verse from "those who have been condemned" which means all who do not follow Islam are not by default under condemned category which would result in punishment in hereafter.
 Muhammad Asad, Message of the Quran, Surah Al Fatiha (Opening) Verse 7
In an authentic hadith, Prophet Muhammad ﷺ warned about five things may lead to five among any people and one of them is related to spread of novel viruses & diseases: “…you may be afflicted by five. God forbid that you should live to see them. If fahisha (immodesty/immorality) should become widespread, you should realise that this has never happened without new diseases befalling the people which their forebears never suffered….” Fahisha means adultery/fornication, indecency, immorality, obscenity, lewdness and pretty much all that is shameful. Allah ﷻ includes zina (adultery/fornication) in fahisha in the following ayah (verse) of the Quran: (Al Quran 17;32) "…And do not go anywhere near zina (adultery/fornication): it is fahisha (immodesty/immorality), and an evil path." Fahisha includes all that brings one closer to zina (adultery/fornication) such as sharing immoral jokes, pictures, shameless talk, lewd speech, all that is shameful and immodest, whether in our speech or actions, that promotes or brings ones closer to the act of zina, even though one may not commit the act itself. Covid-19 is a new disease that was never experienced by our ancestors and if fahisha increases then diseases unknown spread among people, as told by the Blessed Prophet ﷺ. Its quite clear to see that never has the world experienced immodesty at such an unprecedented level as its promoted in our times through advertisements, movies, dramas, music, social media, magazines, news, etc. It is said that 'humans are creature of habits' and sadly, with the spread of latest technology, fahisha has become part of the global culture and habits, and majority has lost the distinction between modesty and immodesty. Hence it is no surprise to see that this novel virus called COVID-19 has engulfed entire humankind. Due to loss of this distinction and becoming used to immorality we have gradually lost the difference between right and wrong in many things in our lives, resulting in becoming the recipient of divine punishment. The Quran teaches that the 1st attack that shaytan (satan) did against humankind was when he made Adam & Eve (alaihim us salaam) uncover their garments:
(Al Quran 7:27–28) Children of Adam! Let not Satan deceive you in the manner he deceived your parents out of Paradise, pulling off from them their clothing to reveal to them their shame. He and his host surely see you from whence you do not see them. We have made satans the guardians of those who do not believe. Yet when [these people] do Fahisha (something disgraceful, shameful), they say, ‘We found our forefathers doing this,’ and, ‘God has commanded us to do this.’ Say [Prophet], ‘God does not command Fahisha (disgraceful, shameful) deeds. How can you say about God things that you do not know [to be true]?’
The “forefathers” is a reference to the inherited traditions and lifestyle that they learned from their peers, teachers, guides in cultures in which they are brought up, who have lost the distinction between modesty and immodesty. The lands which especially passed laws to ban hijab (covering), niqab (veil), and other Islamic coverings from the lives of practicing Muslims, for the fear they might influence their non-Muslims friends, neighbors, relatives etc. are now forced by Allah ﷺ, their Creator, to wear such coverings to make them realize that His Ayahs (signs and verses) in the Quran were no joke!
 Ibn Majah - The Signs Before the Day of Judgment - Ibn Kathir
 The Islamic veil across Europe - https://www.bbc.com/news/world-europe-13038095