Passage: Mahavatar Babaji told Lahirī that he was his guru from the past, then initiated him into Kriya Yoga and instructed Lahiri to initiate others. Lahiri wanted to remain with Mahavatar Babaji, who told him instead that he must return to the world to teach Kriya Yoga and that "Kriya Yoga sadhana would spread through the people of the world through his (Lahiri's) presence in the world."Lahirī reported that Mahavatar Babaji did not give his name or background, so Lahiri gave him the title "Mahavatar Babaji." Many sadhus in India are called Babaji, and sometimes even "Babaji Maharaj", which has caused confusion between Mahavatar Babaji and other sadhus with similar names.Lahirī had many meetings with Mahavatar Babaji, recounted in several books, including Paramhansa Yogananda's Autobiography of a Yogi, Yogiraj Shyama Charan Lahiri Mahasaya (Lahiri's biography), and Purana Purusha: Yogiraj Sri Shama Churn Lahiri, among others.

Question: What type of activity did Mahavatar introduce to Lahiri?
Kriya Yoga