The word “covenant” (3:15) is the Bible word for a contract. When Paul said he was speaking “after the manner of men,” that means he was comparing the covenant God made with Abraham that he’d been talking about (3:7-14) with the contracts men make with each other. Paul makes comparisons like that because of the infirmity of our flesh (cf.Rom.6:19), our infirmity being that we need comparisons like that to better understand Bible truth. Preachers call them sermon illustrations, and they are not only Biblical, they’re Pauline!
Paul begins the comparison by reminding us that contracts have to be confirmed. This is done among men either verbally, with a promise to keep your word, or with a written contract that is confirmed when both parties sign it. Paul reminds us that once a covenant is confirmed, no man can disannul it. Disannulling a contract means to make it null and void, and that’s something you can’t do unless both parties agree to it. And you can’t add to a contract either!
So let’s talk about what the Abrahamic covenant was, and how it was confirmed. On God’s part, He promised to make Abraham’s seed a great nation (Gen.12:2,3) and give them “all the land” of Israel (13:15,16). But for Abraham and the seed of his great nation to possess the land forever, they would have to live forever, so God was promising them that too—all in exchange for nothing but Abraham’s faith in God’s promise to give those things to him and his seed.
Abraham confirmed the covenant when he believed. But how did God confirm it on His part? He swore an oath that He would keep His promise, because He knew that that’s what men expected from each other (Heb.6:13-17). And to get to Paul’s point here in Galatians 3, once both parties confirmed the covenant, it couldn’t be disannulled. That wouldn’t happen, for God wouldn’t break His word, and Abraham would never relinquish eternal life, of course.
But to get to Paul’s other point in this passage, once the covenant was confirmed, God couldn’t add the law to it either. He would never tell Abraham’s seed, “Now you have to keep the law to be saved in addition to just believing Me.”
Next, in verse 17, Paul has to clarify who Abraham’s seed is, for the legalizers thought God promised them eternal life just for being his physical seed. Paul addresses that error by pointing out that Abraham’s ultimate seed was the Lord Jesus Christ. God raised Him from the dead and gave Him eternal life so He could live in the land forever (v.17cf.Acts 2:29-32). That’s a legitimate argument, since “seed” can be singular or plural, like “deer” and “moose” (cf.Deut.22:9).
But the legalists would argue that Genesis 17:7,8 proves God made those promises to the “generations” of Abraham’s seed—and He did! But not his physical generations, his spiritual generations, his believing seed, like the seed that will serve Christ in the kingdom. Psalm 22:28-30 says God will count that seed “for a generation,” and since Christ was a son of Abraham, that’s also the seed Paul is saying God made the covenant with. They are the righteous seed that will live forever in the land in the kingdom (Ps.37:28,29).
This is the same point the Lord had to make to the Pharisees (Jo.8:33-40). Later in Galatians 3, Paul will explain how Gentiles like the Galatians who have Abraham’s faith are also included in God’s promise of eternal life (Gal.3:29).
Galatians 3:17 also tells us it was “God in Christ” who confirmed the covenant with Abraham, i.e., God the Son. If God added the law to that contract, it would nullify it (Rom 4:13,14). And if He nullified the Abrahamic Covenant, that would have made Him a covenant-breaker. And that makes Him a sinner (cf. Rom.1:29). It makes Him a sinner equal with the antichrist, who will confirm the Abrahamic Covenant with Israel, probably by swearing an oath to uphold it (Gen.12:3), and then break it (Dan.9:27).