The main sources of information on the life and teachings of Krishna are the Mahabharata, the Harivamsa, the Vishnu Purana and the Bhagavata Purana and the post Bhagavata literature.
The earliest mention of Krishna, the son of Devaki, is contained in the Chandogya Upanishad, where he is described as the pupil of one Ghora Angirasa, from whom he is said to have learnt that the real sacrifice is the life of man himself – strikingly similar to that taught in the Gita. Chandogya Upanishad belongs to the 6th century BC.
The next mention of Sri Krishna is in Panini, who refers to Vasudeva and Arjuna as objects of worship. Panini’s Ashtadhyayi belongs to the 4th century BC.
The above two sources are only bare mentions. Our earliest source of information for Sri Krishna’s life is the Mahabharata itself. But there it is only his later life that is given – not his early life in the Vrindavan. For this we have to go to Harivamsa and Vishnu Purana. The information contained in these two books is greatly expanded and treated with marvelous power and beauty in the famous tenth Skandha of the Bhagavata Purana.
The Bhagavata Purana subsequently gave rise to five systems of theology in which Krishna is the central object of worship – those of Madhava, Vishnuswami, Nimbarka, Vallabha and Chaitanya – not to speak of the innumerable songs, poems, and dramas in the various vernaculars of India.
The life and teachings of Sri Krishna was discovered from 6th century BC to 16th century AD. But even today many seekers of truth are inspired by teachings of Sri Krishna and find new meanings and information.