The answer is pretty simple, but I'll elaborate, because I think this is really imprtant. A careful reading of the words makes it clear that the warning against adding to or taking away does not refer to the whole Bible or even to the New Testament, but to use John’s words, only to the words of “the book of this prophecy.” That is, the prophecy contained in the book of Revelation. This is substantiated by the fact that some of the books of the New Testament had not yet been written when John wrote the book of Revelation, and even those that had been written and were in existence at that time had not yet been gathered into the bible as we know it.
It is also interesting to note that John himself added to scripture after writing the book of Revelation, which is generally conceded to have been written while he was on the Isle of Patmos.
In the Old Testament also are found similar warnings and commands that there shall not be taken away or added to the words that were written. The first is found in Deuteronomy, written at the time Moses was exhorting Israel to live the law of the Lord. Moses wrote:
“Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the Lord your God which I command you.” (Deut. 4:2.)
Later in this same book of the law, Moses repeated the admonition in similar words. He said,
“What thing soever I command you, observe to do it: thou shalt not add thereto, nor diminish from it.” (Deut. 12:32.)
If the same interpretation and argument was applied to them as is applied to the closing verses of the book of Revelation, there would be no scripture after the writings of Moses.
A distinctive sign of the last days that will precede the eventual second coming of the Lord was seen in vision by that same Apostle who recorded the book of Revelation. He said:
“I saw another angel fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people.” (Rev. 14:6.)
The fact that John saw a messenger from God reveal anew the Gospel of Christ speaks volumes against the interpretation given by some modern Christians.
Hope this helps understand our view.
Sevenbak
And the Mormons take it entirely out of context and use it in a strained eisegetical manner in order to attempt to prove that the gospel was restored by Joseph Smith.
From Albert Barnes' Commentary:
Having the everlasting gospel - The gospel is here called everlasting or eternal:
(a) because its great truths have always existed, or it is conformed to eternal truth;
(b) because it will forever remain unchanged - not being liable to fluctuation like the opinions held by people;
(c) because its effects will be everlasting - in the redemption of the soul and the joys of heaven. In all the glorious eternity before the redeemed, they will be but developing the effects of that gospel on their own hearts, and enjoying the results of it in the presence of God.
The gospel did not need to be restored. It is perpetual. It is, like God, the same yesterday, today, forever. It was never taken from the earth.
If fact, in Mormon theology, John the Apostle never died and is still walking the earth. If John the Apostle never died, and he held the priesthood, as you guys claim, then the priesthood was never taken away, unless it was stripped from John the Apostle and the 3 Nephites. Now maybe the LDS Church saw the contradiction there and maybe they don't teach that anymore. I wouldn't be surprised. They do have a tendency to change their theology as often as some people change their underwear.