For the time will come when they will not endure sound doctrine; but after their own lusts shall they heap to themselves teachers, having itching ears; And they shall turn away their ears from the truth, and shall be turned unto fables.—2 Timothy 4:3-4, New Testament
The Church of Jesus Christ of Latter Day Saints teaches that for a long stretch of History starting at 34AD and lasting until 1820, Christianity underwent something known as The Great Apostasy, which is described as being a mass falling away of the faith, which caused Christianity to deform into different denominations over time. Beginning with the ascending of Jesus Christ to the Celestial Kingdom and ending with Joseph Smith's first vision. During this period, many important doctrines and ordinances essential for salvation were lost from the Earth due to fundamental disputations and misunderstandings amongst early church leaders. Also, new doctrines and pagan traditions were adopted on top of the lost principles. Everytime disagreements occured, splits in churches would occur to suit the individualistic doctrinal desires, the holy priesthood was then lost. Thus, the Great Apostasy was a loss of principle and guidance from the lord on Earth, and was all part of his plan so that he could in the Latter Days, establish his church in an even greater light to the world, and entrench it to make sure that such misguidance could never happen again
Some Christians find the concept of the great apostasy difficult to accept, but the concept is not unique to Latter Day Saints. The protestant movement and Jehovahs witnesses accept that the original church became flawed and mislead over time. But despite that, none of them ever had the pure authority and calling from God to restore the lost doctrine, which was given unto Joseph Smith.