Pharaoh's stubbornness has come to this: God is going to kill the firstborn of every household of Egypt. The Israelites, however, will be protected from harm, because they will have slaughtered a lamb and marked their doors with his blood. Seeing it, God will pass over their houses.
This is the origin of the Passover, when God saves his people through the blood of a lamb. That lamb had to be "without blemish," because it was an image of Christ, the true Lamb of God, who takes away our sins through the shedding of his blood on Good Friday.
"And the Lord said to Moses and Aaron in the land of Egypt: This month shall be to you the beginning of months: it shall be the first in the months of the year. Speak ye to the whole assembly of the children of Israel, and say to them:"On the tenth day of this month let every man take a lamb by their families and houses. But if the number be less than may suffice to eat the lamb, he shall take unto him his neighbour that joineth to his house, according to the number of souls which may be enough to eat the lamb. And it shall be a lamb without blemish, a male, of one year: according to which rite also you shall take a kid. And you shall keep it until the fourteenth day of this month: and the whole multitude of the children of Israel shall sacrifice it in the evening. And they shall take of the blood thereof, and put it upon both the side posts, and on the upper door posts of the houses, wherein they shall eat it. And they shall eat the flesh that night roasted at the fire, and unleavened bread with wild lettuce. You shall not eat thereof any thing raw, nor boiled in water, but only roasted at the fire: you shall eat the head with the feet and entrails thereof. Neither shall there remain any thing of it until morning. If there be any thing left, you shall burn it with fire.
"And thus you shall eat it: you shall gird your reins, and you shall have shoes on your feet, holding staves in your hands, and you shall eat in haste: for it is the Phase (that is the Passage) of the Lord.
"And I will pass through the land of Egypt that night, and will kill every firstborn in the land of Egypt both man and beast: and against all the gods of Egypt I will execute judgments: I am the Lord. And the blood shall be unto you for a sign in the houses where you shall be: and I shall see the blood, and shall pass over you: and the plague shall not be upon you to destroy you, when I shall strike the land of Egypt.
"And this day shall be for a memorial to you: and you shall keep it a feast to the Lord in your generations with an everlasting observance. Seven days shall you eat unleavened bread: in the first day there shall be no leaven in your houses: whosoever shall eat any thing leavened, from the first day until the seventh day, that soul shall perish out of Israel. The first day shall be holy and solemn, and the seventh day shall be kept with the like solemnity: you shall do no work in them, except those things that belong to eating.
"And you shall observe the feast of the unleavened bread: for in this same day I will bring forth your army out of the land of Egypt, and you shall keep this day in your generations by a perpetual observance. The first month, the fourteenth day of the month in the evening, you shall eat unleavened bread, until the one and twentieth day of the same month in the evening. Seven days there shall not be found any leaven in your houses: he that shall eat leavened bread, his soul shall perish out of the assembly of Israel, whether he be a stranger or born in the land. You shall not eat any thing leavened: in all your habitations you shall eat unleavened bread."
- Source: Douay-Rheims 1899 American Edition of the Bible (in the public domain)