Joshua 2:21

She herself puts the scarlet-colored sign in her house, through which she is bound to be saved from the destruction of the city. No other sign would have been accepted, except the scarlet-colored one that carried the sign of blood. For she knew there was no salvation for anyone except in the blood of Christ.

In that the sign hangs in a window I think this is indicated: A window is that which illumines the house and through which we receive light, not wholly but enough, enough to suffice for the eye and for our vision. Even the incarnation of the Savior did not give us pure wine and the whole aspect of divinity, but through his incarnation, just as through the window, he makes us behold the splendor of the divinity. For that reason, so it seems to me, the sign of salvation was given through a window.

So, too, with a mystic reference to the shedding of blood, it was a scarlet cord which the harlot Rahab (a type of the church) hung in her window that she might be saved at the destruction of Jericho.

On account of her faith and hospitality, Rahab the harlot was saved. For when spies were sent by Joshua, the son of Nun, to Jericho, the king of the country ascertained that they had come to spy out their land, and sent men to seize them, in order that, when taken, they might be put to death. But the hospitable Rahab receiving them, concealed them on the roof of her house under some stalks of flax. And when the men sent by the king arrived and said, "There came men unto you who are to spy out our land; bring them forth, for so the king commands," she answered them, "The two men whom you seek came unto me, but quickly departed again and are gone," thus not discovering the spies to them. Then she said to the men, "I know assuredly that the Lord your God has given you this city, for the fear and dread of you have fallen on its inhabitants. When therefore you shall have taken it, keep ye me and the house of my father in safety." And they said to her, "It shall be as you have spoken to us. As soon, therefore, as you know that we are at hand, you shall gather all your family under your roof, and they shall be preserved, but all that are found outside of your dwelling shall perish." Moreover, they gave her a sign to this effect, that she should hang forth from her house a scarlet thread. And thus they made it manifest that redemption should flow through the blood of the Lord to all them that believe and hope in God. You see, beloved, that there was not only faith, but prophecy, in this woman.

For her faith and hospitality Rahab the harlot was saved. For when the spies were sent forth into Jericho by Joshua the son of Nun, the king of the land perceived that they were coming to spy out his country, and [he] sent forth men to seize them, that being seized they might be put to death. So the hospitable Rahab received them and hid them in the upper chamber under the flax stalks. And when the messengers of the king came near and said, "The spies of our land entered into your house; bring them forth, for the king so orders," then she answered, "The men truly, whom you seek, came to me, but they departed immediately and are journeying on the way"; and she pointed out to them the opposite road. And she said to the men, "Without a doubt I perceive that the Lord your God will deliver this city to you; for the fear and the dread of you is fallen upon its inhabitants. When therefore it shall come to pass that you take it, save me and the house of my father." And they said to her, "It shall be even so as you have spoken to us. Therefore, when you perceive that we are coming, you shall gather all your folk beneath your roof, and they shall be saved; for as many as shall be found outside of the house shall perish." And moreover they gave her a sign, that she should hang out from her house a scarlet thread, thereby showing beforehand that through the blood of the Lord there shall be redemption for all them that believe and hope on God. You see, dearly beloved, not only faith, but prophecy, is found in the woman.