And it wasn 't a 'fable'; the Romans were excellent at recording and storing events during theCir reign. The Apostle Paul was a Roman Tax collector, for one whose accounts and writings have withstood the ages.
I think you're confusing Paul / Saul with Matthew / Levi. Matthew wrote the first Gospel, probably around 55-60 AD. He was one of "the Twelve." I have studied his Gospel a lot more than the other three. Memorized mo re than 14 of the 28 chapters. His personal history as a tax collector made him an excellent source to record the events of the life of Christ. And to understand His grace in a profound way.
Paul/Saul was a member of the strict rules follower Pharisee's. He originally murdered early members of the church, but went through a miraculous conversion.
The notion that Jesus was some sort of mythology is pretty biased. There is more evidence that Jesus lived than that, say, Caesar did. Every measure of historical reliability when applied to the gospels, easily stands up when compared to something like Caesar's commentaries on the Gallic Wars. There are more copies. There are earlier copies.
And there are non biblical historical references to Jesus from non believers, especially the Jewish historian Josephus. Luke's gospel is especially valuable in terms of historical reliability. Carefully researched, Luke was a brilliant man, a doctor and ended up sharing the life of Paul during the last decade of Paul's life. Luke became Paul's personal physician. So he spent a lot of time with those who could provide him with first hand information.
His second historical book, Acts of the Apostles is the sequel, and documents the early church and how it came to be. Because that books ends with Paul in prison in Rome, but before his beheading execution, and because we know from other sources that Paul died in the first half of the decade of the 60s (the real 60s) and because we know that Luke's gospel preceded Acts, it had to have been written in the before 60 AD.
Those who don't believe that Jesus lived at all, would have a very hard time with several things that we know for sure. One is the beginnings of the Church, which Luke recorded. It makes no sense that the Church came to be, if those original apostles (the 12) who boldly told the story of Jesus and the church began from nothing, to become a force in the Roman empire.
The other issue is the change in the lives of those early church fathers. 11 of the 12 died martyrs deaths. If they did that for what they knew to be a lie, they were the worst sort of fools.
Anyone who questions whether Jesus really lived and was crucified, and has done due diligence in their historical study, is being very biased in their approach to history. There is a lot of that around.
You can believe what you want about Jesus of Nazareth. But to believe that is was a made up fable doesn't fit with any reasonable approach applied to any other historical figure.