The Lord said to Moses and Aaron in Egypt, 2 “This month is to be for you the first month, the first month of your year. 3 Tell the whole community of Israel that on the tenth day of this month each man is to take a lamb for his family, one for each household. 4 If any household is too small for a whole lamb, they must share one with their nearest neighbor, having taken into account the number of people there are. You are to determine the amount of lamb needed in accordance with what each person will eat. 5 The animals you choose must be year-old males without defect, and you may take them from the sheep or the goats. 6 Take care of them until the fourteenth day of the month, when all the members of the community of Israel must slaughter them at twilight. 7 Then they are to take some of the blood and put it on the sides and tops of the doorframes of the houses where they eat the lambs. 8 That same night they are to eat the meat roasted over the fire, along with bitter herbs, and bread made without yeast. 9 Do not eat the meat raw or boiled in water, but roast it over a fire—with the head, legs and internal organs. 10 Do not leave any of it till morning; if some is left till morning, you must burn it. 11 This is how you are to eat it: with your cloak tucked into your belt, your sandals on your feet and your staff in your hand. Eat it in haste; it is the Lord’s Passover.
12 “On that same night I will pass through Egypt and strike down every firstborn of both people and animals, and I will bring judgment on all the gods of Egypt. I am the Lord. 13 The blood will be a sign for you on the houses where you are, and when I see the blood, I will pass over you. No destructive plague will touch you when I strike Egypt.
14 “This is a day you are to commemorate; for the generations to come you shall celebrate it as a festival to the Lord—a lasting ordinance. 15 For seven days you are to eat bread made without yeast. On the first day remove the yeast from your houses, for whoever eats anything with yeast in it from the first day through the seventh must be cut off from Israel. 16 On the first day hold a sacred assembly, and another one on the seventh day. Do no work at all on these days, except to prepare food for everyone to eat; that is all you may do.
17 “Celebrate the Festival of Unleavened Bread, because it was on this very day that I brought your divisions out of Egypt. Celebrate this day as a lasting ordinance for the generations to come. 18 In the first month you are to eat bread made without yeast, from the evening of the fourteenth day until the evening of the twenty-first day. 19 For seven days no yeast is to be found in your houses. And anyone, whether foreigner or native-born, who eats anything with yeast in it must be cut off from the community of Israel. 20 Eat nothing made with yeast. Wherever you live, you must eat unleavened bread.” Exodus 12:1-20
The Lord gave Moses detailed instructions about what would happen when the Spirit passed over the Israelites. The Passover and the Festival of Unleavened Breat are two rituals that Jews celebrate to this day.
Obviously this plague was a serious matter because of all the human deaths that would occur. When they got the word, the Hebrews would be leaving Egypt in a hurry, which is the whole point of eating unleavened bread. It was such a serious ordeal that someone could be cut off from the Israelite community just by eating anything with yeast in it during the Festival of Unleavened Bread.
The most significant part, however, was applying the blood from the animals sacrificed on the door posts. First, it was an act of obedience because in other plagues, the Lord passed over the Israelites. Of course it was more than an act of obedience because if the blood was not applied to their door posts, there might have been a death in that family.
All of this was a foreshadowing of the coming Messiah. It was through the sacrificial male lamb without defects, Jesus Christ, and His resurrection that we were declared righteous before God. When we accept what He did for us, we are then able to commune with Him.