Bhagat Kabir: 530 hymns: (1398 CE to 1518 Common Era/71 years before Guru Nanak) was raised by Muslim parents. Kabir Panthis (followers of Kabir) say that he lived up to the age of 120 years. Bhagat Kabir Das (Kabir is Arabic for "great", and Das is Prakrit for "slave" or "servant"), is widely acknowledged as one of the great personalities of the Bhakti movement in North India. In AGGS among all Bhagats, Kabir's contribution is the largest, 287 Padas in 17 ragas and 243 Slokes. In his poems, he was quick to express the illustrations of moral and spiritual truth in the incidents of everyday life, and many of his similes and metaphors are very striking. Under each raga or musical mode marking a section of the Holy Book, Kabir's hymns appear at the head of Bhagat Bani, a generic name for the works of contributors other than the Gurus. He lived as a householder, abhorred the caste system and religious rituals. He was a saintly apostle of peace, love and unity and a great poet. Kabir believed in inward purity, and was respected by both Hindus and Muslims.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/gurmat-vichaar-project/23874-bhagats-sri-guru-granth-sahib-ji.html

Conclusions:

1. Kabir was a critique of the Vedic philosophy and fossilized beliefs in the very heartland of the Hindu orthodoxy and makes a departure from Sanskrit which was deemed to be a divine language. Kabir preached in the dialect (Khari Boli) of the common folks. He and other Bhagats popularized, liberalized and democratized the Bhakti cult among the down trodden. Vedic philosophy was also criticized by Gautam Buddha before.

2. Kabir refused to use the Divine language (Sanskrit) instead used 'Khari boli' of the masses for his Bani and so did the Buddhists to write the Buddha's philosophy in Pali instead of Sanskrit before.- Guru Nanak also preferred to use the language of the masses (Punjabi) to write his Thoughts/Bani.

3. The Khari boli of Kabir is very similar to the Punjabi of Guru Nanak. In AGGS some verses of Bani of Guru Nanak and that of Kabir started with letters of Punjabi. It means Punjabi (Khari Boli) was known in Banaras. It is worth noting that the Buddhists, Guru Nanak and Kabir and other Bhagats did not use the Divine language (Sanskrit) to write their philosophies, but used the language spoken by the masses to have close contact with them, so that the masses could easily grasp their message. Question arises as to why do the great Gurus/teachers/prophets/seers preferred to use the language of the masses rather than the Divine language (Sanskrit)? It indicates that the Sanskrit was synthesized for the Brahmans to rewrite the Hindu holy books in that language so that only Brahmans could read it and to shackle the masses into ritualism to control them with their own code of conduct. Under these circumstances it is difficult to relate that Sanskrit could be the mother of all Indian languages since the masses were entirely ignorant of this Divine language (Sanskrit). There is possibility that Sanskrit was codified from the spoken language of the people of the Punjab in such a way that an ordinary person could not understand it and to keep it the language of the elite.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=23874

4. Due to virulent criticism of the Vedic Brahmanism and ritualism Kabir became a charismatic religious personality and was popular among the subaltern masses. His sayings became part and parcel of the collective subaltern psyche. So was done by Guru Nanak.

Virinder S. Grewal