Revelation 21:6

The above quote is taken from Chapter 21, verse 6 of the Book of Revelation in the New Testament of the Bible, with the wording from the original King James Version. 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.

The quote is from the bible, a statement from God claiming that He is eternal (through choice of Greek letters, and a more direct statement) and that He will provides life through water. This is the central concept behind project purity, where the scientists believe that pure water will give hope for humanity.

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.

Framed quote in Vault 101
The framed quote in your Dad's office in Vault 101 can be lockpicked during Trouble on the Homefront. Among other things, it contains a Schematics - Rock-It Launcher and 300 Caps.