The Bible is a set of Christian scriptures, consisting of 66 books of the Old and New Testaments (73 in Catholic Bibles).
The Bible item instantly killing Mom, Mom's Heart, and It Lives is a reference to the epilogue of the Binding of Isaac when the Bible hits Mom's head, knocking out or killing her.
In the original Binding of Isaac, using The Bible on Isaac and ??? would kill the player.
This item is one of the only two in the game that work towards 2 transformations (Book Worm and Seraphim). The other one is Magic Mushroom.
Bug! If The Bible is used on Mom with the Broken Remote trinket, it will not kill Mom and will instead teleport Isaac randomly to another room. However, this still unlocks The Halo as if Mom was killed with the Bible.