Revelation 22 says this:
18 I warn everyone who hears the words of the prophecy of this scroll:
If anyone adds anything to them, God will add to that person the
plagues described in this scroll. 19 And if anyone takes words away
from this scroll of prophecy, God will take away from that person any
share in the tree of life and in the Holy City, which are described in
The context makes it clear that this warning only applies to the book of Revelation itself. However, since the book of Revelation occurs last in most Christian Bibles, and the Apostle John was the last of the Apostles to die and the words and writing of the Apostles are given greater weight than those of other believers, it is commonly assumed by many Christians that this prohibition applies to the whole Bible: no more additions. This has the force, not of doctrine, but of a strong impression that most Christians accept, or at least give credence to.