Charles Ellicott Commentary Galatians 4:29

"But as then he that was born after the flesh persecuted him [that was born] after the Spirit, so also it is now." — Galatians 4:29 (ASV)

Persecuted.—The expression used in Genesis 21:9 is translated in our version “mocking.” It seems doubtful whether the Hebrew can really mean more than “playing.” Jewish traditions added that Ishmael took out the child Isaac and “shot at him with arrows under the pretense of sport.” The Arab tribes, Ishmael’s descendants, had always been a thorn in the side of their Israelite neighbors.

Him that was born after the Spirit.—A miraculous agency intervened in the birth of Isaac, and the Christian Church was inaugurated and inspired by the same agency—that of the Spirit. The Messianic reign was realized through the Spirit, and their participation in this reign made all Christians true and spiritual descendants of Abraham.

Even so it is now.—This seems to have special reference to the behavior of the Judaizing party in Galatia, but would also apply to the relations between Jews and Christians generally.