Hebrews 11:20 - Albert Barnes' Notes on the Bible

By faith Isaac blessed Jacob and Esau concerning things to come - see Genesis 27:26-40. The meaning is, that he pronounced a blessing on them in respect to their future condition. This was by faith in God who had communicated it to him, and in full confidence that he would accomplish all that was here predicted. The act of faith here was simply what believes that all that God says is true. There were no human probabilities at the time when these prophetic announcements were made, which could have been the basis of his calculation, but all that he said must have rested merely on the belief that God had revealed it to him. A blessing was pronounced on each, of a very different nature, but Isaac had no doubt that both would be fulfilled.