Joshua 2 - Red thread hung from the window of Rahab's home is a sign to attacking armies that she and her household are to be saved.
Genesis 38:27-30 - Red thread is used to indicate who was to have the designation and privileges of the firstborn among twins.
Exodus 26 - Red thread was woven into the tabernacle curtains and priestly ephod symbolically representing the sacrificial blood of the covenant.
Theologians and Bible students sometimes refer to “the scarlet thread running through the Bible.” By this they mean that the Bible’s theme is Jesus Christ and His sacrifice for the redemption of mankind. The blood of Christ runs throughout the entire Bible, symbolically. It is seen in the animals killed in Eden to provide garments for Adam and Eve, the ram that took Isaac’s place on the altar of Moriah, the Passover lamb, the institution of the sacrificial system, the scarlet rope of Rahab, and the thousands of years of sacrifices performed at the tabernacle and temple. The scarlet thread runs all the way up to John the Baptist’s declaration, “Look, the Lamb of God, who takes away the sin of the world!” (John 1:29) and to the foot of the cross, where Jesus finally says, “It is finished” (John 19:30).