Paul just finished reminding the Galatians that they’d received the gifts of the Spirit by faith without having to keep the law (v.5), gifts that proved they were saved. This happened “even as Abraham” received salvation by faith without the law (v.6) the moment he believed (Gen.15:5,6).
The legalists were citing Moses to insist they keep the law, as they did in Acts 15:1,5, so Paul cites a source legalists revered more Abraham! They would object that a Jew’s salvation could have no bearing on the salvation of Gentiles like the Galatians, so Paul adds in Galatians 3:7 that any man who shared Abraham’s faith was his child (Rom.4:9-13).
Galatians 3:8 says the Scriptures foresaw this. But books can’t foresee things, only men can (Pr.22:3). But God often identifies Himself with His Book (Ex.9:13,16 cf. Rom. 9:17). So Galatians 3:8 is saying God knew He’d justify Gentiles “through” faith without works, just as He justified Jews “by” faith plus works (Rom.3:30). God foreknew that, but He didn’t foretell it in the Scriptures because it was part of the mystery revealed to Paul (Col.1:26,27).
But to prepare for it, God preached the gospel to Abraham, saying the world would be blessed in him (v.8cf.Gen.18:18). But how? There were two answers one the Scriptures fore-told, and one they didn’t. They foretold the nations would be blessed in a son of Abraham (Ps.72:17) in Christ, the ultimate son of Abraham, in the kingdom (72:8,9,11,14,17, 18). But if the nations want Abraham’s blessing of salvation in the kingdom, they’ll have to keep the law (Isa.56:3-7). But according to the mystery, Gentiles are saved without the law, as Paul is pointing out here. The word “faithful” in Galatians 3:9 means full of faith, not full of faith and the law.
The law curses men (3:10) because they can’t keep it perfectly (James 2:10,11). Paul is quoting a verse (Deut. 27:26) where the law itself curses men if they can’t keep much more than just the ten commandments (15-26). They had to continue to keep all 613 commands in the law (Gal.3: 10) or be cursed, i.e., all your life without ever breaking one!
But God doesn’t curse men just so they walk around feeling cursed. He curses men to get them to see they need a Savior! But after the Savior saves you, the law doesn’t stop cursing you. So if you put yourself under the law, you’re going to feel cursed by the law instead of blessed with Abraham.
To prove his point, Paul quotes another verse (Ps.143:2cf. Gal.3:11), then quotes Habakkuk 2:4 to prove that even under the law men could “live” by faith alone. We know “live” means live eternally because in quoting it Paul doesn’t say, “We’re not justified by the law, we’re justified by faith.” He says, “We’re not justified by the law, we live by faith.”
“The law is not of faith” (Gal.3:12) because if you could keep it well enough to be saved, you wouldn’t need faith. But when Paul tries to prove his point by quoting Leviticus 18:5, that verse makes it sound like men could be saved by keeping the works of the law, and so do Nehemiah 9:29 and Ezekiel 20:11,13. Even the Lord sounded like He thought that too (Lu.18: 25-28) even Paul sounded that way (Rom. 2:6,7). But those verses are just saying God is fair. If any man can earn eternal life by never breaking the law, God will give him what he earned! But there are no takers to that offer.
And because there are no takers, all of us need the cross Paul mentions in Galatians 3:13. Christ redeemed us on the “tree” (cf.IPe.2:24) by being made a curse for us, by being made the thing that the law curses sin (IICor.5:21). God did that, “that the blessing of Abraham might come on us” (Gal. 3:14), i.e., the blessing of eternal life. Not through Israel or the law, as Gentiles were saved in the Old Testament, but “through Jesus Christ.” We also get the Spirit God promised Israel as well. What an unfathomable spiritual deal!