Now you, brothers, like Isaac, are children of promise. But just as at that time he who was born according to the flesh persecuted him who was born according to the Spirit, so also it is now.
-- Galatians 4:28-29
This is a slap in the face of the Judaizers and their justification by circumcision heresy. "You think you’re Abraham’s heirs?" Paul says between the lines (and through them previously). "You think you’re repping Isaac? You with all your covenantal zealotry and your legal p's and q's minded and your pharisaical rigamarole and your detached foreskins think you're Israel? You're children of the flesh. You’re not of Isaac; you're of Ishmael."
This is astounding news. Because this is what Paul is saying: It’s the Galatians – Gentiles brought out of hedonistic paganism! with no prior covenantal cred, much less heritage – who are the children of promise, brothers of Isaac!
Isn’t this amazing news? God takes sin-hungry, idol-crazed, Law-ignorant morons like me and you and, through Christ's righteousness, says "You're Israel. You're my chosen people. You're Abraham's heirs."
The only way this isn't good news is if you think you're hot stuff.
And you are not hot stuff.
The heights of profundity and the depths of knowledge max out at this: God loves sinners so much that he forgives and saves them through the life, death, and resurrection of Jesus Christ.