“The Jews’ religion” (v.13) was originally God’s religion. When He gave it to the people of Israel, it was pure and undefiled (James 1:27). But it got so defiled by their dis-obedience that Paul couldn’t bring himself to call it God’s religion. And before Paul got saved, he was part of that defilement, for he “persecuted the church” (v.13).
Many Bible teachers say there’s only one church in the Bible, “the church, which is His Body” (Eph.1:22,23). But the people of Israel were a church in the wilderness (Acts 7:37). Paul persecuted a church we call the kingdom church. We call it that because the Lord promised to give Peter the keys to it (Mt.16:18,19). The keys were keys of knowledge (cf. Lu.11: 52), i.e., a knowledge of the gospel. That’s what determines who gets into God’s church in any dispensation.
Paul “wasted” that church by killing its members (cf. Acts 22:4). And the reason he’s reminding the Galatians about it is to disprove the legalizers who were saying his gospel was “of men” (Gal.1:11,12). Paul’s message was all about Jesus Christ, and anyone who could have given him a message about Christ was afraid to go near him (cf. Acts 9:13). That proved his message wasn’t of men. The proof that his message was of God came when Paul started preaching his gospel, and had to go from being one of the persecutors to being one of the persecuted! Only God can talk a man into that!
We see more proof that Paul’s message was of God when we learn he was making money in the Jews’ religion (Gal.1:14). Only God can make an unsaved religious leader leave his cash flow to preach a message that left him usually broke.