21 Afterward I went into the regions of Syria and Cilicia.
22 And I was unknown by face to the churches of Judea which were in Christ.
23 But they were hearing only, “He who formerly persecuted us now preaches the faith which he once tried to destroy.”
24 And they glorified God in me.
These verses cover a period of about 10 years.
Syria and Cilicia — two adjacent provinces, the former situated immediately north of Palestine, with Damascus in the south, and Antioch, the capital, in the north; the latter, the chief town of which was Tarsus, lay northwest again. the order of the names is probably the order of the importance of the provinces, not the order in which the apostle visited them. In Acts 9:30 Luke records that he went from Caesarea to Tarsus, apparently by sea, since both these places are ports. In these provinces he seems to have continued for about ten years, apparently making Tarsus his headquarter, 11:25, and laboring in the gospel in the northern parts of Syria, those adjacent to Cilicia, and in Cilicia itself, and founding churches, 15:41. The apostle's purpose, however, is not to define the sphere of his labors during these ten years, but to show that it lay far from Jerusalem and from the possibility of contact with the Twelve.— Galatians, by W.E. Vine, page 150-151
were (v.22) — not in original. Supplied by translators. Probably should be "are."
in Christ (v.22) — probably a reminder to the Galatians that, contrary to what the Judaizers said, these churches weren't Jewish, even though made up of Jews, but were something new.
hearing only (v.23) — continuous tense, ongoing
he who formerly persecuted us (v.23) — in Greek "the persecutor"
the faith (v.23) — Paul did preach to Jews that Christ was the Messiah risen from the grave (as did Peter and the 12), the very message that he'd formerly tried to destroy.
destroy = to ravage, to overthrow. Not a completed deed but a continuous action in an attempt to accomplish the end goal.
glorified (v.24) — were glorifying, continuously. They gave the credit for Paul's change to God, not to Paul. The believers in Judea praised God for Paul's teaching, the same message the Judaizers said was wrong.
Paul has given us proof after proof that his message was not the same as that which the twelve had been preaching. Yet the believers in Judea were saying "… [he] now preacheth the faith which once he destroyed." Does this mean that Paul was, after all, preaching the same gospel which the twelve had been preaching? Some men have used this verse to seek to prove that this was the case. Paul used the greater part of Chapters One and Two to prove that his message was different, but this does not mean that what the twelve had been preaching was not true. They had preached Christ as the prophesied King, no risen from the dead. Paul had once denied this and had persecuted those who believed it.
Then one day Christ Himself had appeared to Saul, and he saw that He was alive, and Israel's rightful King. Thereafter, wherever he went, he sought to convince the Jews of that fact. After all, how could they trust Christ as Lord and Savior if they did not know that He was their true Messiah, risen from the dead? And how could they trust Him as the exalted dispenser of the grace that Paul presented, if He were an impostor whose dead body now lay in a Judean grave? Paul confirmed what Peter and the twelve had been preaching. In that sense, and to that degree, he preached the faith which once he had destroyed.
But even though he confirmed the message of the twelve, never once do we find him preaching the good news of the Kingdom — the good news that the Kingdom was about to be established. He never, like Peter, offered the Kingdom and the return of Jesus Christ if the Jews would repent.
The time for that was past. The establishment of the Messianic kingdom is, even yet in our own day, being held in abeyance until a future time; meanwhile, God offers reconciliation to his enemies by grace through faith. Studies in Galatians, by Cornelius R. Stam, page 95-96