Behold, when we come into the land, you shall bind this line of scarlet thread in the window which you did let us down by: and you shall gather to you into the house your father, and your mother, and your brothers, and all your father's household.
God spared the lives of Rahab and her household because of her faith. Any of Rahab’s relatives that would have gathered with her before the Israelite siege would have done so because of their faith in God’s promise through the spies. If they had no faith they would have stayed in their own homes. Thus the deliverance of Rahab and her family depended on believing a promise from God. Salvation always depends on believing a promise from God (cf. Genesis 15:6; John 3:16; et al.).