In the first Gospel proclaimed today, Jesus rides into Jerusalem in advance of the Passover celebration. The feast of the Passover goes back to ancient Israelite history, which we see recounted in The Books of Genesis and Exodus. The patriarchs of Israel settle in Egypt, where the Israelites start to prosper greatly over several generations. But in time Pharaoh, the ruler of Egypt, turns against them and starts to oppress them to the point of enslaving them.
God then sends Moses and Aaron to tell Pharaoh to let the Israelites leave Egypt. Pharaoh refuses, so God punishes Egypt with ten plagues, the last of which involves the angel of death going through Egypt and striking down the firstborn male of every household. The Israelites can escape this punishment by sacrificing a lamb to be eaten and smearing its blood on the doorframe of their home. The angel of death will then see the blood and pass over the house, sparing that family from the punishment to be inflicted.
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