Monica had at least three children who survived infancy. The oldest, Augustine, is the most famous. At the time of his father's death, Augustine was 17 and a student of rhetoric in Carthage. Monica was distressed to learn that her son had accepted the Manichean heresy – which was a combination of gnostic Christianity, Buddhism, Zoroastrianism, and various other elements, with the basic doctrine of a conflict between light and dark, with matter (physical things) being regarded as dark and evil. At this point, Augustine was living an immoral life. For a while, Monica refused to let him eat or sleep in her house. Then one night she had a vision that assured her Augustine would return to the faith. From that time on she stayed close to her son, praying and fasting for him. In fact, she often stayed much closer than Augustine wanted.
When he was 29, Augustine decided to go to Rome to teach rhetoric. Monica was determined to go along. One night he told his mother that he was going to the dock to say goodbye to a friend. Instead, he set sail for Rome. Monica was heartbroken when she learned of Augustine's trick, but she still followed him. She arrived in Rome only to find that he had left for Milan. Although travel was difficult, Monica pursued him to Milan.
In Milan Augustine came under the influence of the bishop, St. Ambrose, who also became Monica's spiritual director. She accepted his advice in everything and had the humility to give up some practices that had become second nature to her. Monica became a leader of the devout women in Milan, as she had been in Tagaste.
She continued her prayers for Augustine during his years of instruction. At Easter, 387, St. Ambrose baptized Augustine and several of his friends. Soon after, his party left for Africa. Although no one else was aware of it, Monica knew her life was nearing the end. She told Augustine, "Son, nothing in this world now affords me delight. I do not know what there is now left for me to do or why I am still here, all my hopes in this world being now fulfilled." She became ill shortly after and suffered severely for nine days before her death.
O Lord, who through spiritual discipline didst strengthen thy servant St. Monica to persevere in offering her love and prayers and tears for the conversion of her husband and of Augustine their son: Deepen our devotion, we beseech thee, and use us in accordance with thy will to bring others, even our own kindred, to acknowledge Jesus Christ as Savior and Lord; who with thee and the Holy Spirit liveth and reigneth, one God, for ever and ever. Amen.