"Paul wasn't an Apostle of Jesus. He was an enemy" ~Mumuni

You are right that as Saul, he was not an apostle, but an enemy of Christ and His followers. However, in Acts 22, Paul gives the testimony of how he went from enemy to apostle because of God's divine intervention. Since that time, Paul signed his letters "Paul, an apostle of Christ Jesus by the will of God" (Ephesians 1:1) and Peter himself (the disciple Jesus entitled "the rock on which I will build my church") says to listen to Paul who was given wisdom by God and equates Pauls writings with the rest of God-breathed scripture (2 Peter 3:15-17). Paul as well as all of the disciples and many others died painful deaths because the believed Jesus was and is God. Though some people may be willing to die for what they believe to be true and may be mistaken, no one would die for what they know to be false and made up themselves.

I hope this helps. Please feel free to give me feedback or ask any other questions. If any of this has had an impact on your view of God or Jesus, please read this article with an open heart and let me know if there's anything more I can do to help you.