The audience of the letter of Galatians are Gentiles who have responded to God by believing in Jesus as savior. Paul says that they are justified, right with God as a result of that faith. This experience is not unlike that of Abraham, who believed God and “it was credited to him as righteousness” (3:7-9). In the second part of Galatians. Paul turns to a biblical argument, focusing on the phrase “credited as righteousness” in story of Abraham in Genesis 15.
How did “scripture foresee that God would justify the Gentiles by faith” (Gal 3:8)? The Abrahamic covenant states that the whole world would be blessed by the seed of Abraham (Gen 12:1-3). How the nations would be blesses is left unstated, but we know from Galatians that it is through the death and resurrection of Jesus that nations are able to participate in the blessing of Abraham’s covenant.
It is quite possible that there were many Jews living at the time of Paul who would have disagreed at this point, but most would have agreed that God would do something to bring the Gentiles into his future kingdom. The only disagreement was on the percentage of the nations who would respond when the Messiah comes. For some, the nations would come streaming to Zion (Isa 25:6-8), but for others, very few Gentiles would enter the Kingdom. A book like 4 Ezra, for example, doubts if many of the Jews will enter the Kingdom!
But it is highly unlikely that any of the existing Jewish groups as we know them would have expected God to justify the Gentiles by faith, apart from the works of the Law. This is the contribution of Paul: the Gentiles can be right with God without converting to Judaism or keeping the Law or by practicing the boundary markers of Judaism.
The fact that Gentiles would be blessed by the seed of Abraham should not therefore be a surprise to the Jewish church. What is a shock to the Jewish Christians is that Gentiles are to be justified apart from the Law. This was unanticipated in the Hebrew Bible. Abraham therefore becomes the model for the Gentiles since he too was a Gentile, saved by faith in God and not works of the Law.