Ebenezer Erskine, Works, 1:527-528:
You would know, that the gospel, strictly taken, is a word of promise. The first gospel that ever was preached to our first parents, when a dismal cloud of wrath was hanging over their heads in Paradise after the fall, was in a promise, Gen. iii. 15: “The seed of the woman shall bruise the head of the serpent.” The gospel preached to Abraham, what was it but a promise of Christ? “In thy seed shall all the nations of the earth be blessed,” Gal. iii. 8. And I think it observable, that the same thing which the apostle calls the gospel, ver. 8, he calls the promise, and the covenant, ver. 17—19. So that the gospel, strictly taken, is a word of promise: so Heb. iv. 1, 2, compared, — what the apostle calls “a promise of entering into God’s rest” in the 1st verse, he calls the gospel in the 2nd verse. And a God of love and grace dispenses his grace in a promise, for our encouragement to take hold of it in a way of believing; for there is nothing in which the faithfulness of God is so much engaged as in a promise, the very design of which is to be believed.