Tamar was only pretending to be a prostitute when she became pregnant by her father-in-law, Judah, and bore a son who wore the scarlet thread. But we'll see the scarlet thread again...this time, hanging from the window of a real prostitute:
Rahab was a prostitute in the city of Jericho, the first city that Israel encountered on her sojourn from the wilderness into the Promised Land. When Israel sent spies to scope out the city, it was Rahab who sheltered them and kept them from being captured. She believed the rumors that she'd heard about God. (And we saw on Monday this week how she was one spared in an ever-broadening circle of God's all-encompassing plan.)
The spies made an arrangement with the prostitute and when they returned with the full army of Israel, they looked for the scarlet thread hanging in her window. This was the sign that this was her house, that she remembered them and the promise they'd made to her, that she believed them, and that she had done what they asked and was prepared, with her family, to be saved.
Really, the two parties could have chosen any sign. She could have left a lantern burning in plain sight. Or perhaps she could have placed a certain pot on the window sill. Maybe she and her family could have gone up on the roof and waited to be brought down. She could have hung the rope that she let the spies down with out the same window they'd escaped from.
But they chose a scarlet thread.
Now, here's what's cool about the Rahab story. We're told that because of her fear of the Lord, she saved not only herself, but her whole family. The spies told her she was welcome to bring them into her home, and anyone in her home would be saved. So we know that her loved ones were there with her. This makes her the saving grace of her family.
Or, in Israel's terms, a kinsman-redeemer (literally: family-saver).
So then, trace Rahab's descendants a little ways. Rahab, the prostitute, the most despicable woman in all of Jericho, who was the only one to fear the Lord and protect Israel as she came into her Promised Land, married an Israelite - from the tribe of Judah - named Salmon. She gave birth to a son named Boaz.
Boaz is the most famous kinsman-redeemer in all of the Bible. He is the man who married Ruth in the book of Ruth, the closest relative to the daughter-in-law of Naomi, whose husband and sons had all died at a terribly young age. He is the man who provided for Ruth in his fields, then brought her home and married her, giving her (and Naomi) an heir to continue the deceased men's line in Israel's history. That son, that heir, was Obed. Obed was the father of David.
Then we trace David's line all the way down to Jesus, who goes to the Cross as the firstborn Son, the second Adam, the brother of all brothers, Son of God, Son of Man...
...kinsman-redeemer to all who call upon the name of the Lord.