To give a human example, brothers: even with a man-made covenant, no one annuls it or adds to it once it has been ratified. Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many, but referring to one, “And to your offspring,” who is Christ. This is what I mean: the law, which came 430 years afterward, does not annul a covenant previously ratified by God, so as to make the promise void. For if the inheritance comes by the law, it no longer comes by promise; but God gave it to Abraham by a promise. —Galatians 3:15–18
Paul’s response in Galatians 3:15–18 is this: There are among men (v. 15) and between God and man unalterable pacts. God made one with Abraham and his offspring. The pact was that the inheritance of salvation would come, not to all Abraham’s descendants, but to the seed, which is ultimately the Christ, and all who are in him. No Christ, no inheritance! Given the nature of God and his pact, no later stipulation could annul it or void the promise of this pact. Therefore (v. 17), in the law (given 430 years later) God is not putting the inheritance on a new basis. He is not saying: “Once I taught you to trust me; now I teach you to work for me; once I taught you to rely on grace, now I teach you to earn merit; once I taught you to magnify me through childlikeness, now I teach you to magnify yourselves through legalism.” NO! God does not contradict his covenant in this way. He does not commend contrary ways of salvation. If God had set the inheritance on a new basis and taught people to earn their salvation, he would have opposed the promise and nullified grace and promoted pride and cancelled the stumbling block of the cross. The law is holy and just and good; it does not teach us to engage in the Galatian heresy, legalism; it teaches the obedience which comes from faith and applies the Abrahamic covenant to a new stage of redemptive history.