One way out
The world had turned completely to evil. God told Noah a flood was coming, and gave him exact instructions for a vessel that would carry his family through it.
There would be one ark. Not a fleet. One way to be saved.
A door in the side
The ark had one door. And God told Noah exactly where to put it:
"Thou shalt make a window in the ark... and the door of the ark thou shalt set in the side."
Genesis 6:16
In the side. Around AD 420, St Augustine pointed out what that meant: the door in the side of the ark is a picture of the side of Christ, opened by the spear on the cross, through which everyone who is saved enters (City of God 15.26).
Jesus said it himself: "I am the door. If any one enter by me, he shall be saved" (John 10:9).
Sealed with atonement
God also told Noah how to seal the ark against the water:
"Make thee an ark of timber planks: thou shalt make little rooms in the ark, and thou shalt pitch it within and without."
Genesis 6:14
In Hebrew, the word for "pitch" is kopher, and "to pitch" is kaphar. It's the same root as the word for atonement, used for the Day of Atonement, Yom Kippur.
The ark that saved them from judgement was covered, inside and out, with atonement.
God shut the door
Noah, his wife, his three sons and their wives went in. Eight people. And then:
"And they that went in, went in male and female of all flesh, as God had commanded him: and the Lord shut him in on the outside."
Genesis 7:16
Noah didn't close the door. God did. Those inside were kept safe by God himself.
Saved through the water
Peter, writing to the first Christians, explains what the ark was really about:
"...in the days of Noe, when the ark was a building: in which a few, that is, eight souls, were saved by water. Whereunto baptism being of the like form, now saveth you also... by the resurrection of Jesus Christ."
1 Peter 3:20 to 21
The water that destroyed the old world carried the ark into a new one. Baptism is the same: you pass through the water and come out into new life.
And Jesus warned that the end of the world would come like the flood, to people who weren't watching: "as in the days before the flood, they were eating and drinking, marrying and giving in marriage, until the day that Noe entered into the ark" (Matthew 24:38).
Why this is a big deal
One ark. One door, set in its side. Sealed with the word for atonement. God shutting the door himself. Eight people saved through the water.
Every detail of the oldest rescue story in the Bible points to the cross: one Saviour, whose side was opened, through whom everyone who enters is saved.