For I testify unto every man that heareth the words of the prophesy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: And if any man shall take away from the words of the book of this prophecy, God will take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.

Revelation 22:18,19

Though, fairly, most people misrepresent this quote. They do not realize that contextually speaking it has nothing to do with the entirety of the Bible, as the Bible had not been put together at the time of writing. So, John is specifically speaking of his current letter as 'this book'.

However, some can say God divinely moved John to write of something he did not know about. As we all know, that's horseshit. And since most DO believe in God, this argument has no bearing on their divine inspiration assumption.