Why is it that Paul never referred to Jesus as God, but only as Son of God?
It is not true that Paul did not call Jesus God. In Titus 2:13 Paul called Jesus God. Why he usually called him Son of God is that is what Jesus is. Some people claim that Paul did not believe Jesus was God. This claim is proved wrong by Titus 2:13 and by other passages such as Colossians 2:9. “In him the fullness of deity lived in bodily form.