Albert Barnes Commentary Galatians 3:18

"For if the inheritance is of the law, it is no more of promise: but God hath granted it to Abraham by promise." — Galatians 3:18 (ASV)

For if the inheritance. The inheritance promised to Abraham. The sum of the promise was that he should be the heir of the world. See Romans 4:13.

To that heirship or inheritance Paul refers here, and says that it was an essential part of it that it was to be by virtue of the promise made to him, and not by fulfilling the law.

Be of the law. If it is by observing the law of Moses, or if it comes in any way by the fulfilling of law. This is plain. Yet the Jews contended that the blessings of justification and salvation were to be by virtue of observing the law of Moses. But if so, says Paul, then it could not be by the promise made to Abraham, since there could not be two ways of obtaining the same blessing.

But God gave it to Abraham by promise. That, says Paul, is a settled point. It is perfectly clear, and that is to be held as an indisputable fact that the blessing was given to Abraham by a promise.

That promise was confirmed and ratified hundreds of years before the law was given, and the giving of the law could not affect it. But that promise was that he would be the ancestor of the Messiah, and that in him all the nations of the earth should be blessed.

Of course, if they were to be blessed in this way, then it was not to be by observing the law, and the law must have been given for a different purpose. What that was, he states in the following verses.