Mormons believe in the necessity for constant communication with God through the process known as divine revelation:
This principle is basic to our belief. President Wilford Woodruff declared, “Whenever the Lord had a people on the earth that he acknowledged as such, that people were led by revelation” (The Discourses of Wilford Woodruff, sel. G. Homer Durham , 138). We affirm at the beginning that the inspiration of God is available to all who worthily seek the guidance of the Holy Spirit. This is particularly true of those who have received the gift of the Holy Ghost. 
The ninth Article of Faith states the following: “We believe all that God has revealed, all that He does now reveal, and we believe that He will yet reveal many great and important things pertaining to the Kingdom of God.” Thus, Mormons believe in an “open canon of scripture,” allowing the Lord to speak from the past, in the present, and in the future through His prophets. When prophets speak, their words are recorded. Thus, The Church of Jesus Christ of Latter-day Saints adds to the Bible, The Book of Mormon-Another Testament of Jesus Christ, the Doctrine and Covenants (mostly comprised of the revelations of Joseph Smith), and the Pearl of Great Price (revelations of Moses and Abraham). Mormons expect that when the “Ten Lost Tribes” return, they will bring with them their own scripture to add to the body of canonized works, and that other scriptures have been “hidden up by the Lord” to be brought forth at a later day, when men are worthy to receive them. These scriptures have been alluded to in the Book of Mormon and through revelation to modern prophets. They will include the writings of John the Baptist, Joseph who was sold into Egypt, and an ancient prophet called in the Book of Mormon “the Brother of Jared.”
One of the arguments often used in any defense of a closed canon is the New Testament passage recorded in Revelation 22:18: “For I testify unto every man that heareth the words of … this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book.” However, there is now overwhelming consensus among virtually all biblical scholars that this verse applies only to the book of Revelation, not the whole Bible. Those scholars of our day acknowledge a number of New Testament “books” that were almost certainly written after John’s revelation on the Isle of Patmos was received. Included in this category are at least the books of Jude, the three Epistles of John, and probably the entire Gospel of John itself. Perhaps there are even more than these.
But there is a simpler answer as to why that passage in the final book of the current New Testament cannot apply to the whole Bible. That is because the whole Bible as we know it—one collection of texts bound in a single volume—did not exist when that verse was written. For centuries after John produced his writing, the individual books of the New Testament were in circulation singly or perhaps in combinations with a few other texts but almost never as a complete collection. Of the entire corpus of 5,366 known Greek New Testament manuscripts, only 35 contain the whole New Testament as we now know it, and 34 of those were compiled after a.d. 1000.
One Protestant scholar has inquired tellingly into the erroneous doctrine of a closed canon. He writes: “On what biblical or historical grounds has the inspiration of God been limited to the written documents that the church now calls its Bible? … If the Spirit inspired only the written documents of the first century, does that mean that the same Spirit does not speak today in the church about matters that are of significant concern?” We humbly ask those same questions.
One of the great purposes of continuing revelation through living prophets is to declare to the world through additional witnesses that the Bible is true. “This is written,” an ancient prophet said, speaking of the Book of Mormon, “for the intent that ye may believe that,” speaking of the Bible. In one of the earliest revelations received by Joseph Smith, the Lord said, “Behold, I do not bring [the Book of Mormon forth] to destroy [the Bible] but to build it up.”
In a sense Joseph Smith and his prophetic successors in this Church answer the challenge Ralph Waldo Emerson put to the students of the Harvard Divinity School 170 years ago this coming summer. To that group of the Protestant best and brightest, the great sage of Concord pled that they teach “that God is, not was; that He speaketh, not spake.” 
As recorded in the Pearl of Great Price, the Lord said to Moses, ““My works are without end, and … my words … never cease” (Moses 1:4).
Late President and Prophet of the Church, Spencer W. Kimball, declared that the Church continues to be guided by revelation:
“We testify to the world that revelation continues and that the vaults and files of the Church contain these revelations which come month to month and day to day. We testify also that there is, since 1830 when The Church of Jesus Christ of Latter-day Saints was organized, and will continue to be, so long as time shall last, a prophet, recognized of God and his people, who will continue to interpret the mind and will of the Lord. …
“Expecting the spectacular, one may not be fully alerted to the constant flow of revealed communication. I say, in the deepest of humility, but also by the power and force of a burning testimony in my soul, that from the prophet of the Restoration to the prophet of our own year, the communication line is unbroken, the authority is continuous, a light, brilliant and penetrating, continues to shine. The sound of the voice of the Lord is a continuous melody and a thunderous appeal. For nearly a century and a half there has been no interruption.” 
Prophet Joseph F. Smith testified–
The Latter-day Saints … bear testimony to all the world that God lives and that he reveals his will to men who believe in him and who obey his commandments, as much in our day as at any time in the history of nations. The canon of scripture is not full. God has never revealed at any time that he would cease to speak forever to men. If we are permitted to believe that he has spoken, we must and do believe that he continues to speak, because he is unchangeable. …
What is revelation but the uncovering of new truths, by him who is the fountain of all truth? To say that there is no need of new revelation, is equivalent to saying that we have no need of new truths—a ridiculous assertion. As well, too, might we say that the revelations which Abraham received were sufficient for the prophets; that the revelations given to Enoch were sufficient for Noah, whose mission was to build the ark and preach repentance; or that the words spoken to Moses were sufficient for all time; or that what Abraham received would be ample for his children through all the ages. But not so. Notwithstanding Abraham was favored with great promises, the word of God was not denied to his son Isaac, nor to his grandson Jacob. Why? Because these could not have performed their missions on the word of the Lord alone to their father and to others. And how could the Father of the Faithful have accomplished his work on the instructions received by Noah? Of what personal use were the revelations of prior patriarchs and prophets to Balaam or to Paul? It is true, they were of use as historical truths or lessons, but not sufficient for them individually.
So we moderns stand in need, oh so greatly! of constant revelation, that we individually may fill our missions acceptably to our Father, and that we may the better work out our own salvation; and also that we may know the will of God concerning his Church, his people, and his purposes in regard to the nations. These are a few of the thousand needs that exist for revelation. 
Video: Mormons Believe God Speaks
1. James E. Faust, “Continuing Revelation,” Ensign, Aug 1996, 2.
2. Jeffrey R. Holland, “‘My Words … Never Cease’,” Ensign, May 2008, 91–94.
3. Conference Report, Apr. 1977, 115; or Ensign, May 1977, 78.
4. “Editor’s Table: Modern Revelation,” Improvement Era, Aug. 1902, 805–7; paragraphing added.
Go to the next article in the Beliefs section: Mormon Books.