And they shall take of the blood, and put it on the two side posts and on the upper door post of the houses, in which they shall eat it.
All Commentaries on Exodus 12:7 Go To Exodus 12
Augustine of Hippo
For why would the Lord instruct them to kill a sheep on this very feast day except that it was he about whom it was prophesied: “As a sheep is led to the slaughter.” The doorposts of the Jews were marked with the blood of a slaughtered animal. Our foreheads are marked with the blood of Christ. And that sign, because it was a sign, was said to keep the destroyer away from the houses marked with the sign. The sign of Christ drives the destroyer away from us insofar as our heart receives the Savior.