Галатам 3:18

the inheritance--all the blessings to be inherited by Abraham's literal and spiritual children, according to the promise made to him and to his Seed, Christ, justification and glorification (Gal 4:7; Rom 8:17; Co1 6:9). but God, &c.--The Greek order requires rather, "But to Abraham it was by promise that God hath given it." The conclusion is, Therefore the inheritance is not of, or from the law (Rom 4:14).

The apostle in this chapter, I. Reproves the Galatians for their folly, in suffering themselves to be drawn away from the faith of the gospel, and endeavours, from several considerations, to impress them with a sense of it. II. He proves the doctrine which he had reproved them for departing from - that of justification by faith without the works of the law, 1. From the example of Abraham's justification. 2. From the nature and tenour of the law. 3. From the express testimony of the Old Testament; and, 4. From the stability of the covenant of God with Abraham. Lest any should hereupon say, "Wherefore then serveth the law?" he answers, (1.) It was added because of transgressions. (2.) It was given to convince the world of the necessity of a Saviour. (3.) It was designed as a schoolmaster, to bring us to Christ. And then he concludes the chapter by acquainting us with the privilege of Christians under the gospel state.

3:18 The inheritance (see 4:1-7) cannot come through keeping the law, because that would nullify God’s promise to Abraham, which was accepted by faith before the law.