In the book of Joshua we see the story of some spies who sneak into Jericho in order to scout out the city for an upcoming assault. But they were found out, and the city officials were looking for them. To make matters worse, they closed the city gates, and the spies were trapped within the massive, impenetrable walls of Jericho. Thankfully, they found refuge in the house of a harlot named Rahab, who hid them within her house, and eventually let them down the city walls by a scarlet cord.
But, she had a request of the men as payment for her assistance; when the time came for them to attack the city, spare the lives of her and her family. The men agreed, but asked Rahab to bind the scarlet cord to her window, never to remove it, and when they attacked, they would be "passed over" and "saved" by the "blood" red cord.
Do you see the parallels here? This is an amazing story of salvation by the blood! Just like Rahab's home, if we are bound to that blood red cord, then we are saved! But it doesn't just stop with us; the blood can also affect our entire household!