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