(15-18) To take an illustration from purely human relations. A covenant once ratified is binding. It cannot be treated as if it did not exist, neither can fresh clauses be added to it. Now the covenant and promise made to Abraham (by the terms in which it was made) could point to no one but the Messiah. That covenant remained unaffected by the Law, which was four hundred and thirty years subsequent to it in point of date. Law and promise are two totally different and mutually exclusive things. But the covenant with Abraham was given by promise. The Law, therefore, had nothing to do with it.
Related Commentaries of Galatians 3:15
Galatians 3:15-18
15 Brethren, I speak after the manner of men; Though it be but a man's covenant,b yet if it be confirmed, no man disannulleth, or addeth thereto.
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.
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.
18 For if the inheritance be of the law, it is no more of promise: but God gave it to Abraham by promise.