At that time a great persecution arose against the church which was at Jerusalem; and they were all scattered throughout the regions of Judea and Samaria, except the apostles. And devout men carried Stephen to his burial, and made great lamentation over him.
As for Saul, he made havoc of the church, entering every house, and dragging off men and women, committing them to prison. Acts 8:1-3, NKJV
Do you know that just because we sincerely believe a thing, that doesn't make it true? When we're children, tooth fairies, Santa, the Easter bunny, monsters in the closet, boogiemen under the bed--those can all feel very real. Our love of Santa or the terror over the boogiemen can be equally honest beliefs.
Saul honestly thought the Christians were evil, perverting the true religion of the One God of Israel. He was wrong, but that's what he believed and practiced.
Thank the Good Lord, Jesus got through to him. To Saul's credit, when faced with the truth he changed his belief. He became instead a huge proponent and promoter of Christianity.
Now there are hundreds of different belief systems within the Christian religion. Just because you believe a thing doesn't make it true. Check it out, especially with the New Testament. Read the Bible not just once but over and over. Yes, you will find things that conflict--like who saw Jesus first after the resurrection--but much more that agree--like Jesus really did rise from the dead, and He really does love you, and all you really have to do to be forgiven is ask. Well, provided you really do mean you're sorry.
I invite you to challenge your belief system. Find out what Jesus considered important and apply it to your life. It really is of life or death importance!