The night the destroyer passed
Exodus 12 is the foundational scene of OT redemption. Israel has been in Egypt 430 years (Ex 12:40). Nine plagues have failed to move Pharaoh. The LORD announces the tenth: at midnight on the 14th of the first month, every firstborn in Egypt will die — Pharaoh's son, the cattle in the field, the firstborn of the prisoner in the dungeon. Israel is told how to survive it.
Each household selects a lamb on the 10th, holds it four days, slays it on the 14th, and applies its blood to the doorframe:
"And they shall take of the blood, and strike it on the two side posts and on the upper door post of the houses, wherein they shall eat it." (Ex 12:7)
"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." (Ex 12:13)
The Hebrew ot (token / sign / mark) is the same word the prophets use for any visible covenant marker — the rainbow (Gen 9:12), circumcision (Gen 17:11), the Sabbath (Ex 31:13). In each case the ot is a visible thing pointing to an unseen reality. In each case the marked are claimed by the God who marks them.
The geometry of the mark
Exodus 12:22 specifies the application: a bunch of hyssop dipped in the blood, struck on the lintel and the two side posts. Three contact points. With the natural drip of blood from each point, the resulting figure is a four-pointed mark:
──── lintel ────
│ │
left right
post post
│ │
▼ (drip) ▼ (drip)
Some Christian readers (Tertullian, Against Marcion 3.22; Justin Martyr, Dialogue with Trypho 40 and 91; Cyprian, Treatise on the Unity of the Church) extend the line of blood downward through the threshold and read the four-point figure as a cross. The page does not insist on this — Exodus does not say "and the blood formed a cross" — but it is one of the oldest Christian readings of the passage, and it is consistent with the pattern Ezekiel and Revelation will repeat: a household marked, a destroyer coming, the marked spared.
The lamb behind the blood
The blood by itself is not sufficient; it is the lamb's blood, slain that night, eaten that night, "without blemish, a male of the first year" (Ex 12:5), "neither shall ye break a bone thereof" (Ex 12:46). The NT will pick up every clause. Paul names Christ "our passover" (1 Cor 5:7). Peter names him "a lamb without blemish and without spot" (1 Pet 1:19). John names the unbroken-bones detail at the cross: "These things were done, that the scripture should be fulfilled, A bone of him shall not be broken" (John 19:36, citing Ex 12:46 and Ps 34:20).
Hebrews 11:28 anchors the whole scene as a faith act: "Through faith he kept the passover, and the sprinkling of blood, lest he that destroyed the firstborn should touch them." The marked-and-spared pattern is, in the NT's reading, the original instance of justification by faith.
What changed for the unmarked
Egypt was given the same plague-of-the-firstborn warning Israel was. The blood was the difference, not the ethnicity. Exodus 12:48–49 explicitly opens the Passover to "the stranger" who joins himself to Israel by circumcision — "one law shall be to him that is homeborn, and unto the stranger that sojourneth among you." The mark is not racial. It is faith expressed in obedience. The destroyer passes over what is marked.