Commentaries:
Notice, the Gentile converts to the Christian church would attend services from time to time in the Jewish synagogues. Maybe they were the only places they could attend services, and they would hear the law of Moses preached there. The council at Jerusalem had no problem at all with that!
By the time of Galatians, the Gentiles there were being deceived into accepting a strange mixture of Gnosticism and Judaism as the religion of the New Covenant. These Gnostic Jews defined their relationship with God through the law—but law to them was not the same thing as law to a true, God-fearing Christian. When we think of "law," we immediately think of God's law, perhaps specifically the Ten Commandments or generally the Pentateuch. Maybe some of us would think of all the instruction of God, which is really what Torah includes. But "law" to the people deceiving the Galatian Christians was Halakha.
This should not seem strange to us because hundreds of millions of people today call themselves "Christian" yet believe that the law is done away. This happens as a result of hearing something said often enough until it is assumed to be true. In the same way, the Jews honestly and sincerely believed that Halakha was the law of Moses.
Just as important to them, however, is that law was their means of election with God—that is, they believed that the very fact that they possessed the law (Halakha), combined with the quality of their law-keeping, motivated God to choose them. This idea, of course, circumvents God's exercise of His sovereignty over His creation, and is thus false.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part Twenty-Five)
Other Forerunner Commentary entries containing Acts 15:21:
Ephesians 2:8-10