The scarlet cord was meant to be a sign to the Israelites of where the house of Rahab was so that she and her family could be spared. Some people interpret this sign typologically as a reference to Christ, saying that the colour scarlet refers to blood and therefore signifies that only through blood can atonement for sins be made.
1 However, the term for scarlet is used elsewhere in the Bible to describe ordinary things such as the textiles used to decorate the tabernacle (Exodus 25:4), cleansing rituals (Leviticus 14:4), a bright colour (Genesis 38:28), and special garments for the wealthy (Proverbs 31:21).2
The scarlet cord could link Rahab with Tamar, who was also a non-Israelite (see Genesis 38:1–30) and also appears in Jesus’ genealogy (Matthew 1:3–5).3 A better parallel is to compare the purpose of the scarlet cord with the blood on the doorposts during Passover (Exodus 12:1–51). The blood on the doorposts was meant to mark out the people of Israel so that they would escape God’s judgment. The scarlet cord on Rahab’s window will do the same for her.
18 Behold, when we come into the land, you shall tie this scarlet cord in the window through which you let us down, and you shall gather into your house your father and mother, your brothers, and all your father’s household.