The blood shall be a sign for you... and when I see the blood, I will pass over you.
A New Calendar
'This month shall be for you the beginning of months.' Redemption resets the clock. Israel's year will now begin not with creation or harvest but with deliverance — the night God brought them out. Grace reorders time itself; the people of God ever since have kept calendars that begin at salvation.
The Lamb Without Blemish
Each household takes a lamb — without blemish, a male a year old — keeps it four days, and kills it at twilight. The instructions are domestic and exact. This is not temple liturgy yet; there is no temple. It is a family huddled around a sacrifice, eating with sandals on, staff in hand, ready to move when God moves.
The lamb's blemishlessness matters. What stands in for your life cannot itself be forfeit. The New Testament will press the point: 'a lamb without blemish or spot' (1 Pet 1:19) — and the Baptist's cry, 'Behold, the Lamb of God who takes away the sin of the world,' lands with the whole weight of this night behind it.
When I See the Blood
The blood on the doorposts does not inform God — he knows who dwells where. It marks faith made visible: a household sheltering under a death not its own. Judgment falls on Egypt's firstborn; it passes over every door marked by the lamb. Israel is not spared because Israel is innocent. Israel is spared because Israel is covered.
Here is substitution in its oldest biblical form — not yet a theory, but a doorway you stand behind. The firstborn lives because the lamb died. Centuries later Paul writes it in three words and expects the whole picture to rise: 'Christ, our Passover lamb, has been sacrificed' (1 Cor 5:7).
A Memorial Forever
'This day shall be for you a memorial day.' The meal is commanded before the deliverance happens — God institutes the remembering in advance. Every generation eats as if it had been there: 'It is because of what the LORD did for me when I came out of Egypt.' At his last Passover, Jesus took that memorial and bent it around himself: 'Do this in remembrance of me.' The oldest meal in Scripture became the newest.
Go deeper: The Lamb (Symbol Index) · Latreia — the worship word (Lexicon) · Baptism (Disputed Questions)