However, when already a well-established practitioner of Tantra, he underwent a change of heart and made a decision to renounce the search for magical power. Wishing to develop compassion and selflessness, at the age of thirty he took Buddhist vows. Wishing to study with the master of compassion Dharmakirti (Tibetan: Serlingpa), he traveled to the faraway land of Suvarnadvipa (present-day Sumatra). He stayed there for twelve years, learning, among many other things, the Mind Training practice. Such was Atisha's gratitude to Dharmakirti that he was unable even to hear his name without bursting into tears.

On his return to India, Atisha taught for fifteen years at different monasteries and was recognized as both the most learned and the most personally realized teacher in all India. He started to receive invitations to teach in Tibet, which he initally refused. (Tibet at that time had an enormous hunger for true Buddhist teaching but an almost total lack of reliable teachers, due to the brief but severe persecution of Buddhism by the insane King Langdarma.)

Once, in his role as head of discipline at the Vikramsila monastery, he concurred in the expulsion of a monk for drinking alcohol as part of a tantric ceremony. The goddess Tara, his yidam, then came to him in a dream and said that he was responsible for the expulsion of a sincere practitioner, and that as penance he should go to Tibet and teach. The next time the Tibetans invited him, he accepted. The story is told that he had heard that the Tibetans were very open and friendly, so that he would have no-one to challenge him in his compassion practice. So he took along his sulky, bad-mannered Bengali tea boy so that he would have someone to stimulate him in his Mind Training.

He had difficulty getting permission to go from the head of Vikramsila, since his prestige in India was so great. Eventually was allowed to go to Tibet only on condition that he return in three years. However, the need for him and his teaching in Tibet was so great that he never returned, but died there twelve years later.

The information in this biography is mainly taken from Atisha and Tibet,by Alaka Chattopadhaya. There are a lot of uncertainties in this history, fully explained by Mr Chattopadaya, which the above account glosses over.