Albert Barnes Commentary Joshua 2:18

"Behold, when we come into the land, thou shalt bind this line of scarlet thread in the window which thou didst let us down by: and thou shalt gather unto thee into the house thy father, and thy mother, and thy brethren, and all thy father`s household." — Joshua 2:18 (ASV)

The “line” or cord was spun of threads dyed with cochineal: that is, of a deep and bright scarlet color. The color would catch the eye at once and supplied an obvious token by which the house of Rahab might be distinguished.

The use of scarlet in the Levitical rites, especially in those more closely connected with the idea of putting away sin and its consequences (compare, for example, Leviticus 14:4, Leviticus 14:6, Leviticus 14:51; Numbers 19:6), naturally led the fathers, from Clement of Rome onward, to see in this scarlet thread, no less than in the blood of the Passover (Exodus 12:7, Exodus 12:13, and similar passages), an emblem of salvation by the Blood of Christ—a salvation common alike to Christ’s messengers and to those whom they visit.