Imaan (Faith) & Islam (Self-Submission)
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