To begin with we would need to mention the famous incident that took place at Ghadeer Khumm, in which Prophet Muhammad ﷺ brought Hazrat Ali (a.s) closer to him, in front of a large Muslim gathering and said: “Anyone whose Mawla (protecting friend/spiritual guide) I am, Ali is his Mawla (protecting friend/spiritual guide).” Shahwaliullah Dehlavi and Imam Ahmad Sirhindi, 2 renown figures of Islam from Mughal Era India have given an interesting explanation of this narration by mentioning that there are 2 types of leadership or caliphate in Islam, and how did they succeed the blessed Prophet ﷺ.
There are 2 types of Khilafat (Caliphate) - Apparent and Hidden
Shah Waliullah Muhaddith Dehlavi said that two types of leadership appeared after the Prophet; one was the ‘apparent leadership’ (khilafate zahiri) also called the political leadership and the other ‘hidden leadership’ (khilafate baatini) also referred to as wilaya (spiritual sovereignty and leadership). The first Prophet's successor and Caliph Hazrat Abu Bakr inherited the ‘apparent leadership’ (khilafate zahiri), established with the consensus of the companions and majority Muslims, and is categorically proved by the evidence of history and its blessings filtered down to the righteous and just rulers. This is the political office of the religion of Islam, established for improving the administration of the earth.
Shah Waliullah then said that the second type of leadership that emanated from Prophet Muhammad ﷺ was the spiritual legacy of ‘internal sovereignty’ or khilaafate baatini. This spiritual legacy of the Prophet also called wilaya (spiritual sovereignty and leadership) is exclusively a spiritual office, being not an elective and consultative issue but a selective act, selected by Allah ﷻ. The declaration of the first Imam of spiritual sovereignty was done by the Prophet ﷺ at Ghadir Khumm (Pond) of Khumm and Hazrat Ali (r.a) was selected for that honor.
This spiritual leadership that Ali Ibn Abi Talib (a.s) was honored with was to further descend down his chosen ones among his and Fatima (r.a) descendants through their beloved sons Imam Hassan (r.a) and Imam Hussain (r.a). If one notices that majority of the saints that have lived either in the South Asian and Central Asian parts of the Muslim world or North Africa and Middle East were usually descendants from these 2 Imams. This can also be realized when one understands 2 traditions of the Prophet ﷺ in the light of the above explanation when he said “I am leaving two things in your midst if you hold fast to it you will not go astray. They are The Book of Allah and my Sunnah (way & words)" and the next one was "I am leaving two important things in your midst which exceed each other in importance: One (is) Allah’s Book and the other (is) my progeny." The Prophet ﷺ connected the path of guidance to his progeny and descendants who have been given this great burden of carrying over their shoulders of guiding others through knowledge of Allah’s book and Prophetic Sunnah. Many of his descendants did precisely what was predicted by the Prophet ﷺ in the tradition and were able to spread the fragrance of spiritual status that Ali (r.a) received from the Prophet Muhammad ﷺ at the point of Ghadir Khumm.
However one thing to keep in mind is that the way of Ali ibn Abi Talib (a.s) was not only inherited by his descendants but were also inherited by those who were not his descendants. Such as Maruk Karkhi (r.a) received spiritual guidance from Imam Ali Rida (r.a), a descendant of Imam Hussain (r.a) but then became a great saint in his own right. Imam Sirhindi of Mughal era was a great saint and so was Sahl Tustari (r.a), Mawlana Rumi (r.a) and Junayd Baghdadi (r.a) were all great saints of their times and in their own right, were not descended from Ali (r.a) but became great only due to following the path of Imam Ali (r.a), whose life was the embodiment and prime example of Jihad e Nafs (struggle against one's evil self). Thus we notice that majority of Sufi tariqahs (paths) are all inheriting their spiritual way of life from Ali (a.s). Hence comparing the life of Imam Ali (r.a) to the lives of the Sufi saints that have lived throughout the Islamic history, one can notice that they were closer to his path than any one else.
Looking back at the lives of all famous Muslim Sufis we notice that they never preached that there was no difference between Islam and other religions. We do not find any evidence from their lives where they abandoned any one of the 5 pillars of Islam such as salaah or saum (fasting) and adopted other religious principles such as those of Hinduism and Buddhism and or Christianity. Never do we find a renown Muslim Sufi saint who adopted other faiths and its practices and mixed them with those of Islam or even abandoned Islam. However some people have mistook their beautiful style of preaching and tolerance towards other faiths and their followers with that they believed there was no difference at all between Islam and other religions with their erroneous teachings.
The main focus of these pious and saintly ones was actually to stress universal view of God whom Muslims call Allah ﷻ, the Jews call Yahweh, the Hindus call Bhagwan and similarly other cultures and religious backgrounds refer to Him with different terms but that we are all worshiping, in our own ways, the same Deity. They believed that all humanity accepts and worships Him in a manner adopted through interpretation of their texts and teachings. This approach of Sufis towards others promoted tolerance which brought many in to the fold of Islam. This same attitude was also exhibited by the Sultans & Caliphs of Islamic Empires towards other religions and faiths that resided within their domains.
This approach of Sufis is not independent of what Islam teaches. In fact this understanding which is the source of tolerant behavior towards other faiths is rooted in the message of the Quran and the ahadith of Prophet Muhammad ﷺ. For instance verses such as these bear witness: (Al Quran 2:62) "The [Muslim] believers, the Jews, the Christians, and the Sabians- all those who believe in God and the Last Day and do good- will have their rewards with their Lord. No fear for them, nor will they grieve." or (Al Quran 29:46) "Say, ‘We believe in what was revealed to us and in what was revealed to you; our God and your God are one [and the same]; we are devoted to Him."
Prophet Muhammad ﷺ said in his ahadith: “The believers in their mutual kindness, compassion and sympathy are just like one body. When one of the limbs suffers, the whole body responds to it with wakefulness and fever". “O people, be aware: your God is One. All men are the children of Adam and Adam had been created by Allah from clay. No Arab has any superiority over a non-Arab, and no non-Arab any superiority over an Arab, and no white one has any superiority over a black one, and no black one any superiority over a white one, except on the basis of taqwa (piety/God consciousness). The most honorable among you in the sight of Allah is he who is the most pious and righteous of you.” Such prophetic teachings are found echoed in the preaching and poetry of many Muslim Sufi saints throughout the history such as the famous Sufi poet Saadi Shirazi who said:
“Humans (lit., 'children of Adam') are the limbs of one/the same body,
and are from the same essence in their creation.
When the conditions of the time hurt one of these parts,
other parts will suffer from discomfort/restlessness, as well.
If you are indifferent about the misery of others,
it is not deserving to call you a human being." (Gulistan - Saadi Shirazi)
There is no doubt that much of the missionary preaching done throughout the Islamic history was done by Muslim sufi saints that promoted equality of all humans in the sense that all are descendants of the same father Adam (alaihi salaam) and that their Creator is one. However their aim was never to replace Islamic teachings for themselves or their followers, they tolerated other faiths and their practices but never adopted them.
An interesting incident recorded from the life of Ahmad ibn Harb, an early Sufi of Nishapur, with a Zoroastrian neighbor teaches us how these saints interacted and expressed tolerance to wards those of other faiths while simultaneously preaching them the message of Islam. Ahmad ibn Harb had a Zoroastrian neighbor named Bahram. Barham sent a partner out on a trading mission, and on the way thieves had carried off all his goods. Ahmad heard of it and went with his disciples to pay him a visit. “Do not trouble yourself,” Ahmad said. “We have come to sympathize. I heard that your goods had been stolen.” “Yes, that is so,” said Bahram. “But I have three reasons to be grateful to God. First, because they stole from me and not from someone else. Second, that they took only a half. Third, that even if my worldly goods are gone, I still have my religion; and the world comes and goes.” “Write this down,” Ahmad told his disciples. “The odor of Islam issues from these words.” Then he added, turning to Bahram, “Why do you worship this fire?” “So that it may not burn me,” Bahram replied. “Secondly, as today I have given it so much fuel, tomorrow it will not be untrue to me but will convey me to God.” “You have made a great mistake,” commented Ahmad. “Fire is weak and ignorant and faithless. All the calculations you have based on it are false. If a child pours a little water on it, it will go out. A thing so weak as that—how can it convey you to One so mighty? A thing that has not the strength to repel from itself a little earth—how can it convey you to God? Moreover, to prove it is ignorant: if you sprinkle musk and filth upon it, it will burn them both and not know that one is better than the other—that is why it makes no distinction between filth and frankincense. Again, it is now seventy years that you have been worshipping it, and I have never worshipped it; come, let us both put a hand in the fire, and you will see that it burns both our hands. It will not be true to you.” These words struck Bahram to the heart. Bahram then asked few questions about Imaan (faith) to Ahmad and after he heard the answers, he recited the kalimah and became a Muslim.
 Riyad as-Salihin 224 - Sunnah.com
 (Gulistan - Saadi Shirazi)
 Muslim Saints and Mystics, Episodes from the Tadhkirat al-Auliya’ (Memorial of the Saints)
by Farid al-Din Attar, Translated by A. J. Arberry