John Wesley on Genesis 15:17

When the sun was gone down the sign was given - The smoaking furnace signified the affliction of his seed in Egypt: they were there in the furnace of affliction, and labouring in the very fire. They were there in the smoke, their eyes darkened that they could not see to the end of their troubles. 2. The burning lamp speaks comfort in this affliction; and this God shewed Abram at the same time with the smoaking furnace. The lamp notes direction in the smoke; God's word was their lamp, a light shining in a dark place. Perhaps too this burning lamp prefigured the pillar of a cloud and fire which led them out of Egypt. 3. The passing of these between the pieces was the confirming of the covenant God now made with him. It is probable this furnace and lamp, which passed between the pieces, burned and consumed them, and so compleated the sacrifice, and testified God's acceptance of it, as of Gideon's, Jud 6:21, Manoah's, Jud 13:19,20, and Solomon's, 2Ch 7:1. So it intimates, That God's covenants with man are made by sacrifice, Psa 50:5, by Christ, the great sacrifice. God's acceptance of our spiritual sacrifices is a token for good, and an earnest of farther favours.