Revelation 21:6

The above quote is taken from Chapter 21, verse 6 of the Book of Revelations in the old testament of the Bible. It is mentioned several times during the course of Fallout 3 by the protagonist's Father and is said to have been the player's mother's favourite passage.

Fallout 3
After being reunited with their father, the player learns that this passage was the inspiration for Project Purity, an attempt to purify all the irradiated water in the wasteland at once. The passage is quoted several times throughout the story and the chapter/verse number (216) is the code used to activate the water purifier in the game's finale.