NIV Rev 18-21
18I warn everyone who hears the words of the prophecy of this book: If anyone adds anything to them, God will add to him the plagues described in this book. 19And if anyone takes words away from this book of prophecy, God will take away from him his share in the tree of life and in the holy city, which are described in this book.
20He who testifies to these things says, “Yes, I am coming soon.”
Amen. Come, Lord Jesus.
21The grace of the Lord Jesus be with God’s people. Amen.
The Bible wasn't even compiled for 2-3 centuries after these books and epistles were written. Revelations wasn't even written last in the canon. If you apply that statement to the entire Bible, it means that we're not allowed to read anything written by James, Paul, or even the other books written by John.
If you go by your interpretation, then you must also throw out the entire bible after Deuteronomy 4:2-3, since the Lord is commanding pretty much the same thing, not to add to or take away from His word.
Most bible scholars will tell you to be careful how you use that reference from Revelations. And I'd not try to push that agenda here, especially here on a political thread.
Against my better judgment, I will offer one bit of scriptural advice, something I read again today at Church that reminded me a lot of the absurd amount of Mormon bashing that goes on here at FR, and why I think it is so misplaced.
1 John 4:1-3
1 Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.
2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God:
And you certainly don't need to wonder where we stand on the subject of Christ coming in the flesh. It's central to our teachings.
http://scriptures.lds.org/en/gs/j/22
Make it a great day!