For the LORD will pass through to smite the Egyptians; and when he sees the blood upon the lintel, and on the two side posts, the LORD will pass over the door, and will not allow the destroyer to come in unto your houses to slay you.
All Commentaries on Exodus 12:23 Go To Exodus 12
John of Damascus
AD 749
This [cross] we have been given as a sign on our forehead, just as Israel was given the circumcision. For by it we faithful are set apart from the infidels and recognized. It is a shield and armor and a trophy against the devil. It is a seal that the destroyer may not strike us, as Scripture says.