"For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book." (Revelation 22:18)
This sober warning almost at the end of the Bible was given by the glorified Lord Jesus Himself (note v. 20), so should be taken very seriously.
The Bible is not just a great book. It is The Book! Its content had been "For ever . . . settled in heaven" (Psalm 119:89), but had been gradually transmitted to men on earth through God-called prophets, whose writings were "given by inspiration of God"--that is, literally "God-breathed" (2 Timothy 3:16). "God . . . at sundry times and in divers manners spake in time past unto the fathers by the prophets" (Hebrews 1:1).
In various ways--sometimes by direct dictation, more often by Spirit-guided research and meditation using each man's individual style and abilities--God conveyed His message down to mankind. Finally, the New Testament was given through Paul and others "by revelation . . . revealed unto his holy apostles and prophets by the Spirit" (Ephesians 3:3,5). John was the last of the "apostles and prophets" when he wrote Revelation (all the others had been martyred), and Christ then indicated (see text above) that nothing more could be added, not just to John's obviously finished book of Revelation, but really to the now-completed body of inspired Scripture from Genesis to Revelation.
Those "false prophets" (Matthew 24:11) who have tried to add some new revelation to the Bible (e.g., Mohammed) need urgently to go back to the Bible alone. We need no new revelation. The Bible is more than enough for our salvation (2 Timothy 3:15, etc.) and all needed guidance, until Christ returns (2 Peter 1:19). HMM