John Wesley on Genesis 15:18

In that same day, the Lord made a covenant with Abram, saying, Unto thy seed have I given this land - He had said before, To thy seed will I give this land, but here he saith, I have given it; that is, I have given the promise, the charter is sealed and delivered, and cannot be disanulled. The possession is as sure in due time, as if it were now actually delivered to them. In David's time and Solomon's their jurisdiction extended to the utmost of these limits, 2Ch 9:26. And it was their own fault that they were not sooner and longer in possession of all these territories. They forfeited their right by their sins, and by their own sloth and cowardice kept themselves out of possession. The present occupants are named, because their number and strength and long prescription, should be no hindrance to the accomplishment of this promise in its season; and to magnify God's love to Abram and his seed, in giving to that one nation the possession of many nations.