What is the Meaning Behind Thagaste's Response?

With every breath Thagaste leans closer to death's cold. A friend so close, Augustine calls their friendship "sweeter to me than any other sweetness I had known in all my life." In one of of Thagaste's sleep like coma's, he is baptized unconsciously. A flutter of hope, and Thagaste wakes from his sleep-like state. Upon discovery, Augustine pokes fun at Thagaste's baptism. Augustine is expecting a well humored response, instead he receives a harsh and spiteful reply. What is the meaning behind Thagaste's answer. I think the meaning behind this harsh response implies Thagaste was indeed worried about his eternal salvation. Despite the fact that Augusitne had previously lured him away from the teachings of the Bible, perhaps Thagaste was turning towards the Christian faith in life's final hours. I believe that this would have tripped up Augustine. Why would his friend turn to Christ in life's final hours and not toward's Mani's principals? This could have been one of the first  events that caused Augustine turn away from the Manichean teachings.