“Someone is telling me that Galatians 1:23 proves that Paul didn’t get a new message.”
When we teach that Paul had a unique new message, people sometimes respond by pointing out that this cannot be, since it was said of Paul, right after he got saved, that he “preacheth the faith which once he destroyed.” They insist that this means that the message he preached existed before he preached it. However, you’ll notice that Galatians 1:23 says that they “heard” this. Have you ever heard something spoken that turned out later to be untrue? That could easily be the case here.
Remember, Paul’s gospel and the kingdom gospel were both centered in Christ, and so when Paul got saved and began preaching Christ, that could easily explain how “the churches of Judaea” (Gal. 1:22) “heard” that Paul preached their faith. They would have had to have heard this second hand, from easily confused unbelievers, since believers were afraid to go near Saul in those early days (cf. Acts 9:26).
If there was any truth to the rumor, then the phrase “the faith” was being used in the generic sense. When this phrase is used by Paul, it refers to the body of truth that was given to him (Acts 14:22; 16:5; Rom. 1:5; Gal. 3:23; Eph. 4:13; Col. 1:23; I Tim. 3:9; I Tim. 4:1; II Tim. 3:8; Titus 1:13). When this phrase is used by Bible writers other than Paul, however, it refers to the body of truth of the kingdom message (Acts 6:7; I Pet. 5:9; Jude 1:3). source