Exodus 12:7,13 (NKJV)
And they shall take some of the blood and put it on the two doorposts and on the lintel of the houses where they eat it.
Now the blood shall be a sign for you on the houses where you are. And when I see the blood, I will pass over you; and the plague shall not be on you to destroy you when I strike the land of Egypt.
The instructions for the first Passover supper must have seemed strange to the Israelites, but they dared not question what God had said he was going to do--they had already seen his mighty hand in the previous nine plagues. A lamb was slaughtered and roasted and eaten and his blood was placed on top and on the two sides of the door to the house in the form of a cross. God was painting a picture, a symbol, a sign of the real Lamb of God who was to come so that we might be found under the protection of His blood. What does the blood do? It satisfies God's wrath against our sin when He sees the blood of the innocent Lamb and allows Him to pass over our sin. Danger is coming but there is protection under the blood.
Romans 3:25-26 (NKJV)
... whom God set forth as a propitiation by His blood, through faith, to demonstrate His righteousness, because in His forbearance God had passed over the sins that were previously committed, to demonstrate at the present time His righteousness, that He might be just and the justifier of the one who has faith in Jesus.