Bhagat Kabir Ji has 541 compositions in the Guru Granth Sahib Ji, more than any contributor other than the Gurus themselves. He was a weaver of Benares, brought up in a Muslim julaha family, and he attacked the ritual apparatus of Hindus and Muslims with equal force.
His presence in a Sikh scripture is not incidental. Guru Arjan Dev Ji put him there in 1604, in the ragas, alongside the Gurus, without commentary. The Sikh position is that truth spoken is truth regardless of who spoke it, and Kabir Ji is the largest single piece of evidence for it.
There is a separate collection, the Kabir Bijak, held by the Kabirpanth, and the Rajasthani manuscripts differ again. The three recensions do not agree, and the version Punjabis know is the Guru Granth Sahib Ji's.
In Punjab his lines are proverbial in ordinary speech among people who have never opened a book. ਕਬੀਰ in the margin of a page is enough to make a village audience listen.
About five hundred of his compositions are in the Guru Granth Sahib Ji, more than any other Bhagat and more than Guru Angad Dev Ji or Guru Tegh Bahadur Ji, spread across the raga sections and in the shloks.
His method is the reversal: taking what the listener holds and turning it over, in short, hard, often abrasive verses that refuse both Hindu and Muslim authority in the same breath.
The Punjabi recension of his work differs from the Bijak of the Kabirpanth and from the Rajasthani collections. Which is closest to what he said is one of the oldest open questions in Indian textual scholarship.