Paul says this in Acts 26:9–11 , when he explains to King Agrippa that he once thought he “ought to do many things against the name of Jesus of Nazareth.”

In that same passage, Paul describes:

  • persecuting believers,
  • trying to force them to blaspheme,
  • and being intensely opposed to the early Christian movement.

A close companion passage is Acts 22:4–8 , where Paul says he persecuted “this Way” and was confronted by the risen Jesus on the road to Damascus.

If you want the exact wording from one Bible translation, I can give that too.