John Calvin Commentary Galatians 3:29

"And if ye are Christ`s, then are ye Abraham`s seed, heirs according to promise." — Galatians 3:29 (ASV)

Then are you Abraham’s seed. This is not intended to convey the idea that to be a child of Abraham is better than to be a member of Christ—but to repress the pride of the Jews, who gloried in their privilege, as if they alone were the people of God. They regarded no distinction higher than belonging to the race of Abraham; and this very distinction He makes common to all who believe in Christ.

The conclusion rests on this argument: that Christ is the blessed seed, in whom, as we have said, all the children of Abraham are united. He proves this by the universal offer of the inheritance to them all, from which it follows that the promise includes them among the children. It deserves notice that, wherever faith is mentioned, it is always its relation to the promise.