Lessons from the past
As the angel of death prepared to pass over the land of Egypt, every firstborn of Egypt and Israel, both people and animals, would be destroyed. This was the penalty of sin. There was only one way to escape. If a lamb without blemish would be sacrificed and its blood smeared on the door posts of the house, the angel of death would pass over. This was true for Jew and Gentile alike - there was no other way to be saved. This was a blood covenant and as in all blood covenants, two are involved; God and the head of the household. Two elements were also needed - the blood of an innocent lamb and obedience.
It was not enough to just sacrifice the lamb, it's blood had to be applied to the door posts of the house. They had to act in faith that God would honor His word and passover when He saw the blood. About midnight, by candle light, the family was inside the house, eating the entire roasted lamb because God had commanded to leave nothing until the morning. They also ate unleavened bread. No doubt the children were asking what was happening as they heard the screams of those who had not prepared. The fathers began teaching them, so the story would not forgotten from generation to generation.
There are many lessons to be learned here: 1) There is only one way of escape, 2) the sacrifice blood must be applied, 3) obedience is required, 4) faith kept them inside their house - their ark of safety while the destroyer passed over, 5) by faith and obedience and telling his children the head of the house hold can give them a place of safety, 6) they had to eat all of the lamb, nothing was to be left until the morning, 7) they had to be ready and willing to go out of Egypt when the signal sounded.
Christians today have forgotten many of these lessons. It is 'politically incorrect' to say there is only one way to be saved, and it can even be equated with religious 'hate speech'. But the only truth that will deliver you from the coming Day of Jacobs trouble is "the blood must be applied to the door post of your house" to be saved. It is not enough to mentally claim to believe, you must act and actually apply the blood! All of the lamb must be eaten. As the writer of Hebrews declares, "Let us leave the elementary teachings about Christ and go on to maturity, ..." - Hebrews 6:1. Many are unable to rightly understand Scriptures that were given for our protection in the end days, because they are still on milk.
Passover was kept with the Feast of Unleavened Bread because we must search our lives, judging ourselves, and put away worldly things that will just weaken us. We must eat unleavened bread, untainted with sin.