Joshua 2:12 Now, therefore, I pray you, swear unto me by the Lord, since I have showed you kindness, that ye will also show kindness unto my father’s house, and give me a true token.
Rahab responded to the news about the Lord God of Israel by believing in Him as her God, and then her faith response moved her to show kindness to the two spies and save them. She put her money where her mouth was. She evidenced her faith by her works. She didn’t know what the Lord was going to do, that the whole city and everyone in it would be destroyed. But she knew enough to pray that her family be saved. And sure enough, every part of the walls of Jericho was demolished except for Rahab’s house.
The scarlet thread by which the spies escaped was left hanging out of her window. By this token Joshua’s army knew to spare that family. The scarlet thread is a symbol of the blood of Jesus, which is what really spared her and her family. It is like the blood of the Passover Lamb: “When I see the blood, I will pass over that house.” So the blood of Christ stays the just judgment of God on our sin.
Rahab did not show kindness in order to be saved; God did not save her because she was kind. She showed kindness because of her faith. In the same way, we are kind to others because we are already saved and forgiven.