Gill's Exposition explains the scarlet thread: "so the scarlet thread was an emblem of the blood of Christ, by which salvation is; redemption and all the blessings of grace are through it; justification, remission of sin, reconciliation, and atonement, and safety, and protection from avenging justice, and wrath to come, are only by it"... more at end
You may hear Christians speak of the "Scarlet Thread." This is a shorthand reference not only to the literal verse literally reporting on Rahab's scarlet thread, but to the fact that the blood of Jesus and His plan for Redemption runs through all the bible from Genesis to Revelation. The bible is about the atonement from beginning to end, and it's about the One who accomplished that atonement, Jesus. There is no "Old Testament God of Wrath" and a "New Testament God of Love." There is no God vs. Jesus. There are no two halves, one fulfilled and one marginalized. The scarlet thread runs through the entire bible.
Here is an example from the bible about the scarlet thread. God told Hosea His prophet to tell the Israelites this pronouncement of judgment,
The high places of Aven, the sin of Israel,
shall be destroyed.
Thorn and thistle shall grow up
on their altars,
and they shall say to the mountains, “Cover us,”
and to the hills, “Fall on us.”
For those who believe that God is outdated and so is the Old Testament, or for those who believe it's not necessary or study the Old Testament, or for those who believe that the OT has no bearing on our lives today in this modern world, here is Jesus on His way to Calvary referring back to that exact Hosea prophecy and also prophecies in Hosea 9:11, 14, (Give them, O LORD— what will you give? Give them a miscarrying womb and dry breasts. ... Ephraim's glory will fly away like a bird-- no birth, no pregnancy, no conception.")
"And there followed him a great multitude of the people and of women who were mourning and lamenting for him. But turning to them Jesus said, “Daughters of Jerusalem, do not weep for me, but weep for yourselves and for your children. For behold, the days are coming when they will say, ‘Blessed are the barren and the wombs that never bore and the breasts that never nursed!’ 30Then they will begin to say to the mountains, ‘Fall on us,’ and to the hills, ‘Cover us.’ "
And once again the scarlet thread progresses in its weaving truth to men as we reach Revelation, where once again the Hosea prophecy of the past, the Jesus mention at the midpoint, and the reference is made again, this time to the future:
"Then the kings of the earth, the princes, the generals, the rich, the mighty, and everyone else, both slave and free, hid in caves and among the rocks of the mountains.
They called to the mountains and the rocks, “Fall on us and hide us from the face of him who sits on the throne and from the wrath of the Lamb!"
Back to Gill's again on Rahab's Scarlet thread as metaphor in Joshua 2:
and thou shall bring thy father, and thy mother, and thy brethren, and all thy father's household home unto thee; into her house, where the scarlet thread was bound, and where only they would be safe, as the Israelites were in the houses where the blood of the paschal lamb was sprinkled, Exodus 12:23; and so they are safe, and they only, who are under the blood of sprinkling, and partake of the virtue of it.
Praise the Lord for the fact the scarlet thread exists! With it we are covered, safe, and dwelling in the House...'where ONLY they would be safe'! The beauty, wonder, mystery, and truth of God's word is always inspiring and convicting. Jesus' blood runs throughout the entire word, for He IS the Word.