This is all founded on the idea of a great apostasy and its consequences.
James E. Faust, a General Authority, recently said, "I wish to testify today of the fulness of the restored gospel of Jesus Christ, which adds to the religious beliefs of other denominations, both Christian and non-Christian. This fulness was originally established by the Savior in His earthly ministry. But then there was a falling away." (http://lds.org/general-conference/2006/04/the-restoration-of-all-things?lang=eng&query=great+apostasy)
Similarly, Boyd K. Packer said, "The Church you belong to, The Church of Jesus Christ of Latter-day Saints, is the restored Church. 3 When you know what restored means, you will understand why standards of conduct are as they are.Following the Crucifixion of Christ an apostasy occurred. Leaders began to “teach for doctrines the commandments of men.” 4 They lost the keys of authority and closed themselves off from the channels of revelation. That lost authority could not just be repossessed. It had to be restored by those who held the keys of authority anciently. 5The Church of Jesus Christ of Latter-day Saints is not a remodeled version of another church. It is not an adjustment or a correction or a protest against any other church. They have their “form of godliness” 6 and their goodness and value." (http://lds.org/general-conference/2003/10/the-standard-of-truth-has-been-erected?lang=eng&query=great+apostasy)
Thankfully, these claims of a great apostasy and its results can be examined. The idea of a "great apostasy" quite simply doesn't fit into history. Here's a few reasons why.
Oral tradition at that time period was outstanding--there have been studies done that show that the same story could be passed over generations with no significant changes. This is for a few reasons. First of all, memorization skills were more keenly honed, developed, and utilized at this time period. It was not unthinkable for a rabbi or student of the Law to have whole books of scripture memorized. Oral tradition was held as sacred; an elder would quickly correct a younger person who got an important detail wrong. This was not like a game of telephone, where a person only hears it whispered once and has to pass it on. This was a community effort to maintain and pass on tradition and knowledge. The stories of Jesus would not have been ruined by the few decades where it existed only orally.
The Gospels were written either by eyewitnesses or people who had interviewed eyewitnesses. These people had motivation to write it down correctly, and the people who had been there were still around to correct anything that had been written down incorrectly. This contributes at least in part to the consistency of the Gospels, especially the synoptic Gospels (Matthew, Mark, and Luke)--while small details sometimes vary, the core of the stories match extremely well. A lawyer, judge, or other expert in modern law would tell you that this is not only expected in stories where witnesses are telling you the truth, it is also vital, because if the stories were to match 100%, there would be accusations that the witnesses had gotten together to concoct a story, which would then bring its truthfulness into question.
The epistles of Paul were written even earlier--even closer to the events.
The Gospels and epistles were written early enough that there was not enough time for legendary development--the adding in of events that were untrue and often larger-than-life. Studies show that it usually takes much longer than the couple of decades that the writers had at this time, and the people writing still had a vested interest in getting it right. The legendary development can be seen later, in the Gnostic gospels, which is why these are not accepted into canon.
There is plenty of manuscript evidence to show that there were no significant changes in scriptural texts, and enough manuscripts being written that it would have been hard for a group or individual to make a change that would have gone unnoticed and been perpetuated. Even when the Christian faith began to have organized religion--the Catholic church--there were many manuscripts, and it would have been impossible to completely erase the evidence of significantly changing the contents of that many Bibles, especially since some of those early Bibles and many partial manuscripts are still preserved today.
There is no evidence of much of the practices of the LDS church being done in ancient times. Temple marriages were not performed. Baptisms for the dead in the temple were not performed--neither were endowments. There was no hierarchical order where there was a single prophet at the top, with apostles underneath him, and an organization that trickled down from there--any hierarchy was imposed by men. Even in the Old Testament, if often speaks of multiple prophets and female prophets, and there were no apostles at that time. In the New Testament, there were no prophets as leaders after John the Baptist:
Luke 16:16 The Law and the Prophets were proclaimed until John. Since that time, the good news of the kingdom of God is being preached.
While the spiritual gift of prophecy still exists today, there are not prophets in the same way that there were in the Old Testament.
The councils that the LDS church so vilifies were not for men to impose their own views on the gospel, but for what already existed to be made official. The Trinity was made official because of heresies that contradicted the Bible--but the Trinity existed in the gospels and the epistles, and was written of by the earliest church fathers, before the councils ever convened. The canon was not chosen because they were books men wanted in scripture--they took the books generally accepted as scripture and, with strict standards that kept anything incorrect out of the God's Word, compiled those books into the accepted canon that is now the Bible.
There is also Christ's own promises that we can rely on when it comes to the preservation of the Word of God and the true church:
Heaven and earth will pass away, but my words will never pass away.
And I tell you that you are Peter, and on this rock I will build my church, and the gates of Hades will not overcome it.
If Christ promised it and the evidence backs it up, why should we not believe it?
In truth, it is not that the gospel was lost, but that men do not always understand it. There is a reason for some of the things Christ said:
Matthew 7:21 Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven.
Matthew 7:13 Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it. 14 But small is the gate and narrow the road that leads to life, and only a few find it.
The narrow gate is very simple to find, but its so hard for many people:
John 14:6 Jesus answered, “I am the way and the truth and the life. No one comes to the Father except through me."
Romans 6:23 For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.
Romans 10:9 If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved.
We can see that there was no apostasy. The Bible is preserved. No important doctrines were lost. The ordinances and temples that the LDS hold up so high are unnecessary and contradict the Bible. All we need...is Jesus. His Word is sufficient to guide us to him.