In an age of increasing respect of reason, people in Europe began to reject the authority of the Catholic Church, which had for ages administered all things spiritual. Until this time, the Bible had been painstakingly hand-copied by servants of the Church, and only the clergy possessed copies of it. Lay people had to rely on the readings and interpretations of the clergy for access to the Bible, as well as access to the saving ordinances. As religious scholars risked their lives to translate the Bible into vernacular tongues, beginning with Luther’s translation into German, and to get Bibles into the hands of the people, a new authority emerged. The Protestant Reformation sought to do away with the religious authority in Rome and to reform the corrupt and erroneous practices and ideas of the mother church. Once the authority of Rome was displaced, what remained was the Bible and the “authority” of “priesthood of all believers.” In other words, the Bible, as understood by the community of Christ, became the only conduit between man and God.
In the absence of continuing revelation—or even trustworthy counsel from church leadership—reformers turned more and more to their understanding of the Bible. It was a great blessing that the Bible was pushed to center stage to guide the people and settle disputes. Yet it is clear that the Bible was never intended as a comprehensive guide to faith and practice. If that idea were ever in doubt, we only need look at the conflict between the reformers. While all of them took the Bible seriously, each understood it very differently; they could not come to a unity of the faith in spite of their devotion to the Bible. Deep divisions continue to this day. 
The idea that the Bible is perfect and “inerrant” is fairly new, only a couple hundred years old.
For a believer in biblical inerrancy, Holy Scripture is the Word of God, and carries the full authority of God. Every single statement of the Bible calls for instant and unqualified acceptance. Every doctrine of the Bible is the teaching of God and therefore requires full agreement. Every promise of the Bible calls for unshakable trust in its fulfillment. Every command of the Bible is the directive of God himself and therefore demands willing observance. (“Biblical Inerrancy” in Wikipedia)
Along with the idea that the Bible is perfect comes the supposition that it contains everything necessary to lead men and women to salvation and to establish manners of worship. That the Bible is completely sufficient means that nothing else is needed, and indeed, anything extra should be rejected. Floyd Filson, once aprofessor at McCormick Theological Seminary, said the following:
It is God who speaks to man. But He does not do so only through the Bible. He speaks through prophets and apostles. He speaks through specific events. And while his unique message to the Church finds its central record and written expression in the Bible, this very reference to the Bible reminds us that Christ is the Word of God in a living, personal way which surpasses what we have even in this unique book. Even the Bible proves to be the Word of God only when the Holy Spirit working within us attests to the truth and divine authority of what the Scripture says…Our hope is in God; our life is in Christ; our power is in the Spirit. The Bible speaks to us of the divine center of all life and help and power, but it is not the center. The Christian teaching about the canon must not deify the Scripture (1957, pp. 20-21, emphasis added).
The eighth of the Thirteen Articles of Faith of The Church of Jesus Christ of Latter-day Saints says,
We believe the Bible to be the word of God as far as it is translated correctly; we also believe the Book of Mormon to be the word of God.
In fact, Mormons believe there are even more words of God than is contained in these two books, and that there is much still to come forth. The ninth Article of Faith says,
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.
In the Book of Mormon 2 Nephi 29:
…many of the Gentiles shall say: A Bible! A Bible! We have got a Bible, and there cannot be any more Bible.
Know ye not that there are more nations than one? Know ye not that I, the Lord your God, have created all men, and that I remember those who are upon the isles of the sea; and that I rule in the heavens above and in the earth beneath; and I bring forth my word unto the children of men, yea, even upon all the nations of the earth?
Wherefore murmur ye, because that ye shall receive more of my word? Know ye not that the testimony of two nations is a witness unto you that I am God, that I remember one nation like unto another. And when the two nations shall run together the testimony of the two nations shall run together also.
And I do this that I may prove unto many that I am the same yesterday, today, and forever; and that I speak forth my words according to mine own pleasure. And because that I have spoken one word ye need not suppose that I cannot speak another; for my work is not yet finished; neither shall it be until the end of man, neither from that time henceforth and forever.
The Lord Himself has spoken both about the purity and insufficiency of the Bible (1 Nephi 13):
Behold it proceedeth out of the mouth of a Jew. … which contains the covenants of the Lord, which he hath made unto the house of Israel; and it also containeth many of the prophecies of the holy prophets; and …they contain the covenants of the Lord, which he hath made unto the house of Israel; wherefore, they are of great worth unto the Gentiles.
…and when it proceeded forth from the mouth of a Jew it contained the fulness of the gospel of the Lord, of whom the twelve apostles bear record; and they bear record according to the truth which is in the Lamb of God.
…And after they go forth by the hand of the twelve apostles of the Lamb, from the Jews unto the Gentiles, … behold, they have taken away from the gospel of the Lamb many parts which are plain and most precious; and also many covenants of the Lord have they taken away.
…And after these plain and precious things were taken away it goeth forth unto all the nations of the Gentiles; and after it goeth forth unto all the nations of the Gentiles, yea, even across the many waters which thou hast seen with the Gentiles which have gone forth out of captivity, thou seest—because of the many plain and precious things which have been taken out of the book, which were plain unto the understanding of the children of men, according to the plainness which is in the Lamb of God—because of these things which are taken away out of the gospel of the Lamb, an exceedingly great many do stumble…
…because of the most plain and precious parts of the gospel of the Lamb which have been kept back… saith the Lamb—I will be merciful unto the Gentiles in that day, insomuch that I will bring forth unto them, in mine own power, much of my gospel, which shall be plain and precious, saith the Lamb.
And after it had come forth unto them I beheld other books, which came forth by the power of the Lamb, from the Gentiles unto them, unto the convincing of the Gentiles and the remnant of the seed of my brethren, and also the Jews who were scattered upon all the face of the earth, that the records of the prophets and of the twelve apostles of the Lamb are true.
And the angel spake unto me, saying: These last records, which thou hast seen among the Gentiles, shall establish the truth of the first, which are of the twelve apostles of the Lamb, and shall make known the plain and precious things which have been taken away from them; and shall make known to all kindreds, tongues, and people, that the Lamb of God is the Son of the Eternal Father, and the Savior of the world; and that all men must come unto him, or they cannot be saved….And the time cometh that he shall manifest himself unto all nations, both unto the Jews and also unto the Gentiles…
The Lord Jesus Christ in Mormonism