What does Galatians 1:23 mean?
Paul is making the case for why the Judaizers are wrong that he learned about Christ from the apostles. He has shown he wasn't really even around the apostles for the first several years of his ministry.In fact, he was gone from Jerusalem and Judea for so long that the Christians in that region only knew him by his two wildly different reputations: Paul—then called Saul—was originally a Pharisee who vehemently persecuted and tried to destroy the church back when he was in Jerusalem. Those who knew of his exploits would have been shocked to hear that the same man was reported to be out in Syria and Cilicia preaching that people could be saved by God's grace through faith in Christ!
In fact, Paul learned about the gospel of God's grace through Jesus from Jesus Himself. He was, truthfully, an apostle, and he wanted his Galatian readers to know they could trust his message about Jesus.