3:15-18 The covenant God made with Abraham, was not done away by the giving the law to Moses. The covenant was made with Abraham and his Seed. It is still in force; Christ abideth for ever in his person, and his spiritual seed, who are his by faith. By this we learn the difference between the promises of the law and those of the gospel. The promises of the law are made to the person of every man; the promises of the gospel are first made to Christ, then by him to those who are by faith ingrafted into Christ. Rightly to divide the word of truth, a great difference must be put between the promise and the law, as to the inward affections, and the whole practice of life. When the promise is mingled with the law, it is made nothing but the law. Let Christ be always before our eyes, as a sure argument for the defence of faith, against dependence on human righteousness.

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 (Ga 4:7; Ro 8:17; 1Co 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 (Ro 4:14).