“ Behold, when we come into the land, you will bind this line of scarlet cord in the window which you let us down by, and you will gather into your house your father, and your mother, and your brothers, and all your father's household, and it shall be that whoever shall go out of the door of your house into the street, his blood shall be on his own head, and we will be guiltless, and whoever shall be with you in the house, his blood shall be on our head, if any hand be upon him.”
The piece of scarlet cord seems to have been a regular recognised token, possibly a love or commitment token somewhat like an engagement or eternity ring (Genesis 38:28; Genesis 38:30; Song of Solomon 4:3) worn round the neck, which was sometimes used as a guarantee and may have borne a seal so that it was recognisable (compare Genesis 38:18). One of the men handed over his token as their guarantee and commitment that the woman would be secure, along with all who were in the house.
The scarlet thread was to be placed on the window on the wall of the city. It was in some ways similar to the blood of the Passover lamb (Exodus 12), for it would protect from YHWH's avengers. The protection of the building itself was not the original intention for the spies did not know how God would open up the city, but it achieved this as well (Joshua 6:22-23). Note that it was placed on the window on the wall of the city, not on the door of the house, so that in any attack on the walls that area would be spared. This practical note is a sign of authenticity, even though in the event it was not necessary. The walls were not attacked.
The warning that only those who remained within the house would be safe was again similar to the Passover (Exodus 12:22). It was the only way in this case in which the people could be identified. By it they were sanctified (set apart as holy and untouchable) to YHWH under the sign of the scarlet thread.
The two spies stated that they would bear blood guilt if anyone within the house under the sign of the scarlet cord should die. On the other hand any who refused that protection and left the house would bear their own guilt.