Saint Paul wasn't always a follower of Jesus, in fact for a long time Paul actually despised Jesus and all that he stood for.
Saint Paul, at this time known as Saul, was a Greek-speaking Jew and was a Roman Citizen.
Saul was a Pharisee. He believed that Jesus, and in extension Christians were a threat to the Jewish religion and so he sought to persecute them all.
One famous persecution was that of Saint Stephen. Stephen was a diligent follower of Christ and became the first Christian Martyr. He was stoned to death for his beliefs. Apparently Saul was there, but just watched him die.
So how did Saul become Saint Paul? Well, when Saul was on the way to prosecute more Christians. He was stopped by a blinding Light.
In that instance, Saul heard Jesus and Jesus asked him why he was persecuting him! It was at that time that Saul finally realised all that he had done and was blinded.