41 Besides this wonderful agreement between the types and the things typified, the adversary may be convinced by plain prophetic intimations, such as this: "In your seed shall all nations be blessed." This was said to Abraham, and again to Isaac, and again to Jacob. Hence the significance of the words "I am the God of Abraham, and Isaac, and Jacob." God fulfills His promise to their seed in blessing all nations. With a like significance, Abraham himself, when he made his servant swear, told him to put his hand under his thigh; for he knew that thence would come the flesh of Christ, in whom we have now, not the promise of blessing to all nations, but the promise fulfilled.
Source: Reply to Faustus the Manichaean (New Advent)