The conversion of Paul is one of the most fascinating events in early Christian history.  People do not just one day change their beliefs completely, without a dramatic, genuine, influencing event.  No one changes their moral, religious or political beliefs without a real, external, genuine influence. This is what happened to Paul.  Those who had previously known him as a persecutor, frankly, a murderer (of Stephen and perhaps others), believed his conversion was real and due to a personal appearance to Paul of the resurrected Jesus Christ.  Skeptics have never been able to come up with a believable alternative explanation.