Augustine's life has changed in Milan.
While still at Carthage, he had begun to move away from Manichaeism, in part because of a disappointing meeting with the Manichean Bishop, Faustus of Mileve, a key exponent of Manichaean theology.
In Rome, he is reported to have completely turned away from Manichaeanism, and instead embraced the skepticism of the New Academy movement.
His mother, who had followed him to Milan, has pressured him to become a Christian.
Augustine's own studies in Neoplatonism are also leading him in this direction, and his friend Simplicianus has urged him this way as well, but it Ambrose who has the most influence over Augustine.
Ambrose, like Augustine himself, is a master of rhetoric but older and more experienced.
Augustine allows his mother to arrange a society marriage, for which he abandons his concubine.
It is believed that Augustine truly loved the woman he had lived with for so long.
In his "Confessions," he expresses how deeply he was hurt by ending this relationship, and also admits that the experience eventually produced a decreased sensitivity to pain over time.
However, he has to wait two years until his fiancee comes of age, so despite the grief he feels over leaving "The One" as he calls her, he soon takes another concubine.
Augustine eventually breaks off his engagement to his eleven-year-old fiancee, but never renews his relationship with "The One" and soon leaves his second concubine.
It is during this period that he utters his famous prayer, "Grant me chastity and continence, but not yet" (da mihi castitatem et continentiam, sed noli modo).
In the summer of 386, after having read an account of the life of Saint Anthony of the Desert which greatly inspired him, Augustine undergoes a profound personal crisis, which leads him to convert to Christianity, abandon his career in rhetoric, quit his teaching position in Milan, give up any ideas of marriage, and devote himself entirely to serving God and to the practices of priesthood, which includes celibacy.
He will detail his spiritual journey in his famous Confessions, which is to become a classic of both Christian theology and world literature.
Ambrose baptizes Augustine, along with his son, Adeodatus, on Easter Vigil in 387 in Milan.
Eventually named a Latin church father, Augustine is to become one of the most important figures in the development of Western Christianity.
According to his contemporary, Jerome, Augustine "established anew the ancient faith" (conditor antiquae rursum fidei).