Exodus 3: 7, 8, 10; 5:1,2; 12:12 - 14

"And the LORD said, I have surely seen the affliction of my people which are in Egypt, and have heard their cry by reason of their taskmasters; for I know their sorrows; And I am come down to deliver them out of the hand of the Egyptians. . . Come now therefore, and I will send thee unto Pharaoh, that thou mayest bring forth my people the children of Israel out of Egypt. . .And afterward Moses and Aaron went in, and told Pharaoh, Thus saith the LORD God of Israel, Let my people go. . . And Pharaoh said, Who is the LORD, that I should obey his voice to let Israel go? I know not the LORD, neither will I let Israel go."  After nine ?attempts' God told Moses ". . . 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.  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. And this day shall be unto you for a memorial; and ye shall keep it a feast to the LORD throughout your generations; ye shall keep it a feast by an ordinance for ever."

The Children of Israel were God's chosen.  He provided all for them.  Yet, they found themselves in slavery.  Serving a Pharaoh, who did not believe in God.  Serving a Pharaoh, who feared their population growth.  After nine encounters to have Pharaoh to free the Children of Israel, God launched the ultimate plague - death to the firstborn of every living creature; animals and people alike.  This was one stipulation, that the Children of Israel cover their doorpost with the blood of a sacrificed lamb, so their firstborn would be spared.