Final answer:
Augustine believed in the concept of original sin and that divine grace is necessary to resolve the conflict between good and evil.
Step-by-step explanation:
Augustine's favorite sin during adolescence is not explicitly mentioned in the provided information. However, it is important to note that Augustine believed in the concept of original sin, which refers to the inherent sinful nature of humanity passed down from Adam and Eve. According to Augustine, human beings have the capacity to choose good or evil, and divine grace is necessary to resolve the conflict between these rival wills.