Paul tells us explicitly that before becoming a follower of Jesus he was a persecutor of the church. And why was he persecuting it? He doesn’t say directly, but my sense is that it was for a very basic reason. He despised their message. Specifically, he could not abide by what Christians were saying about Jesus. Why was that a problem? Because they insisted he was God’s messiah...
And who was Jesus? He was a crucified criminal. As an insignificant and relatively unknown apocalyptic preacher from a rural part of the northern hinterlands, he made a solitary pilgrimage to Jerusalem with a handful of followers ending up on the wrong side of the law. He was unceremoniously tried, convicted, and tortured to death on criminal charges. That’s the messiah? That’s just the opposite of the messiah.
There are good reasons for thinking that some of Jesus’ followers (it is impossible to say how many there were, but given the demographics of rural Galilee we’re not talking thousands or even hundreds) thought that maybe he would be the messiah. Those hopes were forcefully and convincingly dashed by his execution. But for reasons we do not need to explore here, some of them came to think that after his death a great miracle had occurred and God had brought Jesus back to life and exalted him up to heaven. This belief reconfirmed the earlier expectation: Jesus is the one favored of God! He is the anointed one! He is the messiah!
This reconfirmation of a hope that had been forcefully disconfirmed compelled these earliest followers of Jesus to make sense of it all through their ultimate source of all religious truth, the sacred scriptural traditions. They found passages that spoke of someone (a righteous person or the nation of Israel as a whole) suffering but then being vindicated by God. These included passages such as Isaiah 53 quoted above. These followers of Jesus claimed these passages actually referred to the future messiah. They were predictions of Jesus.
This was for them “good news.” Jesus was the messiah, but not one anyone expected. By raising him from the dead, God showed that Jesus’ death had brought about much greater salvation than anyone had anticipated. Jesus came to save God’s people not from their oppression by a foreign power, but to save them for eternal life. This is what the earliest Christians must have proclaimed.
And for the zealous Pharisee Paul, it was utter nonsense. It was worse than nonsense. It was a horrific and dangerous blasphemy against God, his scriptures, and the law itself. This scandalous preaching had to be stopped. And Paul did his best to stop it. (Ehrman
https://ehrmanblog.org/why-paul-persecuted-the-christians/ )