12 For I will pass through the land of Egypt this night, and will smite all the firstborn in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgment: I am the Lord.

13 And the blood shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and the plague shall not be upon you to destroy you, when I smite the land of Egypt.

Here, at last, God explains the curse He is about to bring upon Egypt and how the blood of the lamb will save the Israelites from it. In many of the prior curses, God also put a separation between the Israelites and the Egyptians, but in each of those cases there was not anything that the Israelites had to do to receive that protection, it just came for granted. This final curse is unique in that the Israelites will still be separated, but only so long as they meet the requirement that God has given to them.

We see this same sort of idea in the Christian theology, where we believe that some gifts come to all, and some don’t. All people are saved from death by the resurrection, and no effort is required of us to receive this free gift, but at the same time Christ calls us to become perfected in him, and do the work that he gives us, that we may enjoy a oneness with him and the Father in heaven. Of course, even in the cases where some effort from the faithful is required, God’s grace is always still a factor. Even though the Israelites did the work of following God’s commands to kill the lamb and put its blood on their posts, there isn’t anything inherently life-saving in those actions. They only worked because God made them work by His grace. It is the same with our efforts to become more like Christ. They only work because of a miracle of transformation that God brings to our hearts.

Also, note in verse 12 that God is not merely doling out His judgment upon the Egyptian people, but also upon their gods. He had made mockery of their pagan beliefs before when He sent the murrain that slew their sacred flocks, and now He had His people slaying those same lambs, eating their flesh, and painting their blood on the doors for all to see. The Egyptians would consider all of these acts a spiritual desecration, yet the Israelites would blessed and preserved for doing it. God would therefore justify what the people of Egypt denounced, and He would denounce what the Egyptian gods had justified.

In short, there were multiple layers of destruction and humiliation facing the Egyptians this night, and when they awoke they would become a profoundly broken people.