Charles Ellicott Commentary Joshua 2:19

"And it shall be, that whosoever shall go out of the doors of thy house into the street, his blood shall be upon his head, and we shall be guiltless: and whosoever shall be with thee in the house, his blood shall be on our head, if any hand be upon him." — Joshua 2:19 (ASV)

Whosoever shall go out of the doors of your house into the street, his blood shall be upon his head. —Compare Exodus 12:22 (the account of the Passover): You shall... strike the lintel and the two side-posts with the blood that is in the basin: and none of you shall go out at the door of your house until the morning; for the Lord will pass through to smite the Egyptians.

What the blood was to the houses of Israel in Egypt, that the scarlet line in the window was to the house of Rahab. Both alike prefigured the precious blood of Christ.