Charles Ellicott 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)

The fulfillment of the promise is unaffected by the Law. For it is not dependent upon the Law, or upon the Law and the promise combined (the Law modifying the promise), but upon the promise alone. The Law does not come in at all.

Law and promise—in other words, contract and free gift—are incompatible ideas. But the land of Canaan was promised to Abraham as a free gift, and as a free gift, the spiritual Canaan is thrown open to his spiritual descendants.

The inheritance. In the first instance, this refers to the temporal inheritance of the land of Canaan; but here it is understood as the spiritual blessings of the Messianic kingdom.

Gave it. The word in the original is strong: God has freely given it. There is an antithesis to the idea of a “covenant” or “contract,” in which both parties have to perform a part. The promise was given by God to Abraham freely, gratuitously, unfettered by any obligation on his part, the non-fulfillment of which might make it void.