12. Now, therefore, I pray you, swear, etc. It is another manifestation of faith that she places the sons of Abraham in sure possession of the land of Canaan, founding on no other argument than her having heard that it was divinely promised to them. For she did not suppose that God was favoring lawless intruders who were forcing their way into the territories of others with unjust violence and uncurbed licentiousness, but rather concluded that they were coming into the land of Canaan, because God had assigned them the dominion of it. It cannot be believed that when they sought a passage from the Edomites and others, they said nothing as to whither they were going. Nay, those nations were acquainted with the promise which was made to Abraham, and the memory of which had been again renewed by the rejection of Esau.
Moreover, in the language of Rahab, we behold that characteristic property of faith described by the author of the Epistle to the Hebrews, when he calls it a vision, or sight of things not appearing. (Hebrews 11:1) Rahab is dwelling with her people in a fortified city: and yet she commits her life to her terrified guests, just as if they had already gained possession of the land, and had full power to save or destroy as they pleased. This voluntary surrender was, in fact, the very same as embracing the promise of God, and casting herself on his protection. She, moreover, exacts an oath, because often, in the storming of cities, the heat and tumult of the struggle shook off the remembrance of duty. In the same way she mentions the kindness she had shown to them, that gratitude might stimulate them the more to perform their promise. For although the obligation of the oath ought of itself to have been effectual, it would have been doubly base and inhumane not to show gratitude to a hostess to whom they owed deliverance. Rahab shows the kindliness of her disposition, in her anxiety about her parents and kindred. This is, indeed, natural; but many are so devoted to themselves, that children hesitate not to ransom their own lives by the death of their parents, instead of exerting courage and zeal to save them.