Galatians 4:21 Meaning

19My little children, of whom I travail in birth again until Christ be formed in you,

20I desire to be present with you now, and to change my voice; for I stand in doubt of you.

21Tell me, ye that desire to be under the law, do ye not hear the law?

22For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman.

23But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise.