John Calvin Commentary Galatians 3:8

"And the scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand unto Abraham, [saying,] In thee shall all the nations be blessed." — Galatians 3:8 (ASV)

The scripture foreseeing. What he had said in a general manner is now applied expressly to the Gentiles, for the calling of the Gentiles was a new and extraordinary occurrence. Doubts existed as to the manner in which they should be called. Some thought that they were required to be circumcised and to keep the law (Acts 15:24), and that otherwise they were shut out from having a share in the covenant.

But Paul shows, on the other hand, that by faith they arrive at the blessing, and by faith they must be in grafted (Romans 11:17, 24) into the family of Abraham. How does he prove this? Because it is said, In thee shall all nations be blessed.

These words unquestionably recall that all must be blessed in the same manner as Abraham, for he is the model, indeed, the rule, to be universally observed. Now, he obtained the blessing by faith, and in the same manner must it be obtained by all.