Why was it not enough for Paul to simply believe and preach that Jesus was the Christ, the Son of God?
It was not enough because God had further revelations to give him. Simply believing and preaching Jesus as the Christ and Son of God was no longer sufficient; the message could not stop there. He had to be prepared, including through suffering and danger, to take the Gospel of Grace.
Les explains that after the man met the Lord on the road to Damascus, was stricken blind, then received food and sustenance and got his sight back, he went into the synagogue and proclaimed that Jesus was the Christ. But that was no longer sufficient. It was not enough to just simply believe that Jesus was the Christ, the Son of God, because God had further revelations to give the man, so He could not leave him with that kind of a message. Word came that people were ready to put him to death, so he had to flee for his life, being let down over the wall in a basket. The governor under Aretas the king kept the city of the Damascenes with a garrison desirous to apprehend him, and he escaped. Les says this life of suffering and fear began right from the beginning and was all part and parcel of getting the man ready to take the Gospel of Grace.