John Calvin on Galatians 4:21

21. Tell me. Having given exhortations adapted to touch the feelings, he follows up his former doctrine by an illustration of great beauty. Viewed simply as an argument, it would not be very powerful; but, as a confirmation added to a most satisfactory chain of reasoning, it is not unworthy of attention.

To be under the law, signifies here, to come under the yoke of the law, on the condition that God will act toward you according to the covenant of the law, and that you, in return, bind yourself to keep the law. In any other sense than this, all believers are under the law; but the apostle treats, as we have already said, of the law with its appendages.