23 They only heard the account: “The man who formerly persecuted us is now preaching the faith he once tried to destroy.” -Galatians 1:23 BSB
The saints in Jerusalem heard of Paul’s intent and eagerness to persecute the other believers in different regions. The church then heard of his conversion and how now he is preaching everywhere about Jesus being the Messiah.
When Saul, also known as Paul, persecuted the church, see verse 13 and its commentary.
For Paul’s conversion, read Acts 9:1-19.