Charles Ellicott Commentary Galatians 4:21

"Tell me, ye that desire to be under the law, do ye not hear the law?" — Galatians 4:21 (ASV)

You that desire to be under the law.—A direct appeal to those who were inclined to give way to the Judaizing party.

Do you not hear the law?—“Hear” is probably to be taken in the sense of “give heed to,” “listen to with attention,” as in Matthew 10:14; Matthew 13:9; Matthew 13:13; Luke 16:29; Luke 16:31. Some have thought that it merely refers to the practice of reading a lesson from the Old Testament, which was adopted into the Christian Church from the synagogue.

On verses 21-31:

The next eleven verses contain an elaborate argument from the history of the two sons of Abraham, as types of the two covenants, in further proof that freedom is the essential character of the Christian dispensation.

We have seen that St. Paul applies the history of the natural Israel allegorically to the spiritual Israel; and not only does he do this with reference to the history of the formed theocracy, but he goes back to its origin in the time of the patriarchs, and traces there the first beginnings of the separation between the Law and the promise. The same history had been already allegorically treated by Philo. The treatment of it by St. Paul is, however, quite different, and in keeping with the line of argument followed in the context.