Your lamb shall be without blemish, a male of the first year: ye shall take it out from the sheep, or from the goats: And ye shall keep it up until the fourteenth day of the same month: and the whole assembly of the congregation of Israel shall kill it in the evening. And they shall take of the blood, and strike it on the two side posts and on the upper door post of the houses, wherein they shall eat it.
Exodus 12:5-7
When Moses gave the Israelites the instructions for the Passover before the night when all the firstborn in Egypt would be killed, he gave them specific detailed directions for how they could be safe. Every family that put the blood of the lamb on the door was spared from the heartbreak and tragedy. Every family that did not suffered the death of every firstborn in the house. It did not matter if they believed the God of Israel was the true God. It did not matter if they believed that Moses had been sent by God to lead them. Their true faith would be seen through whether or not they put the blood on their door posts.
This Old Testament story is a picture for us of the blood of Jesus that atones for our sin. There are many people who profess to believe in God and even believe that Jesus died on the cross and raised from the dead. But if they do not place their faith in Him alone to be their Savior, if they attempt to earn their way to Heaven through their own good works, they have only head knowledge and not real, living faith. James 2:23 says, "Thou believest that there is one God; thou doest well: the devils also believe, and tremble." Knowing the truth about Jesus is not enough; we must put our faith in His shed blood to be saved. Just as the Israelites were to apply the blood of a lamb to their door posts, so we are saved by trusting in the blood shed for us by "the Lamb of God, which taketh away the sin of the world" (John 1:29).