Summary: In O.T. times some Israelites made vows to God. Definition: Vows are commitments to God to perform some duty or abstain from some activity.
Gal 3:15 Brethren, I speak after the manner of men; Though it be but a man's covenant, yet if it be confirmed, no man disannulleth, or addeth thereto.
Gal 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
Gal 3:17 And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect.
Gal 3:18 For if the inheritance be of the law, it is no more of promise: but God gave it to Abraham by promise.
(15-18) The unchanging nature of God's covenant with Abraham.
Brethren, I speak in the manner of men: Though it is only a man's covenant, yet if it is confirmed, no one annuls or adds to it. Now to Abraham and his Seed were the promises made. He does not say, "And to seeds," as of many, but as of one, "And to your Seed," who is Christ. And this I say, that the law, which was four hundred and thirty years later, cannot annul the covenant that was confirmed before by God in Christ, that it should make the promise of no effect. For if the inheritance is of the law, it is no longer of promise; but God gave it to Abraham by promise.
a. Brethren, I speak in the manner of men: Paul first establishes the principle that even with a covenant among men, the covenant stands firm once it is made - no one annuls or adds to it. Paul's point isn't really about covenants among men, but to say "how much more with a covenant God makes!"
i. But we shouldn't miss the first word of Gal_3:15: Brethren. As difficult and dangerous as Paul's opponents in Galatia were, they were also his brothers. He confronts them and persuades them as brothers.
b. Now to Abraham and his Seed were the promises made: In Gen_22:18, God promised Abraham that in your seed all the nations of the earth shall be blessed. Paul observes that the singular for seed is used, not the plural. The point is clear: "And to your Seed," who is Christ. God is referring to one specific descendant of Abraham, not all his descendants in general
c. This covenant and promise was made not only to Abraham, but also to Jesus the Messiah. So we cannot think that it was over-ruled by the covenant God later established with Israel at Sinai.
i. The unchanging nature of God's covenant with Abraham is an important principle; it was a one-sided covenant (Gen_12:1-3; Genesis 15) that was promised forever (Gen_17:7-8). There is no if in Abraham's covenant (Gen_12:1-3; Genesis 15); but there are plenty of "ifs" in the covenant of Moses (Deuteronomy 28). The Mosaic covenant was a two-sided covenant, with two both parties obligated to uphold it, not a one-sided covenant (Exo_24:3-8)
ii. This means that promise of a land (Gen_12:1; 13:7; 15:18-21), a nation (Gen_12:2; 13:16; 17:4-6; 22:17) and of a blessing (Gen_12:2-3; 22:18) stands eternally to Abraham and his descendants, though spiritually the promise of a blessing comes to every nation through Jesus, by faith.