Okay, here is my response to
Lawdog.
The concept of a universal "apostasy" from the truth that occurred after the death of the Apostles is a central tenet of Mormon faith. If this apostasy had not occurred, there would have been no need for a "Restoration" of the truth, which we believe happened through the Prophet Joseph Smith.
Nevertheless, the idea that the earth was entirely thrust into darkness is not true. There were and are good men and women everywhere who strive to follow the teachings of Christ to the best of their ability and understanding, and the Light of Christ was and is given to everyone who comes into the world. Thus the "Great Apostasy," or the "falling away" from the truth which Mormons believe happened, was not such that all truth was lost to the world until Joseph Smith came. Rather, it was a loss of Priesthood authority, which authority is God-given to the faithful in all dispensations.
This idea of a "dispensation" requres some further elaboration. Latter-day Saints tend to recognize seven different major dispensations, or the restoration of gospel truths to mankind after apostasy has occurred: those of Adam, Enoch, Noah, Abraham, Moses, Jesus Christ, and Joseph Smith. Doubtless there have been other dispensations, but those are the major ones. Sometime after each dispensation, apostasy has reared its ugly head and men have rebelled against God, thereby losing Priesthood authority (the authority to act in God's name and with God's power on the earth). When this apostasy has been total, there has to be a restoration of the truth and of the Priesthood keys and ordinances.
Although the "Great Apostasy" occurred, that doesn't mean that all truth was lost from the earth. Good men and women through the ages have kept the lamp of truth lit to the best of their abilities. However, in the case of the Great Apostasy, all Priesthood power and authority was lost from the earth, leaving the Church of Christ in a state of apostate truth. It had a form of godliness, but denied the power thereof. It taught for doctrines the commandments of men, and God Himself said that although they drew near Him with their lips, their hearts were far from Him (speaking of the churches in Joseph Smith's day).
According to Alexander B. Morrison,
"The complexity of Christian history and the sheer lack of definitive information about its crucial early years make it difficult at best to pinpoint the causes of the apostasy that Christianity underwent, to ascertain the changes which represent rejection of God's truth and rebellion against Him, and to determine the teachings and practices that remained true to those of Christ and His apostles. [...]
The record of first-century Christianity has large gaps. Almost no reliable documents from that period survived beyond those now included in the New Testament.... There are many things we simply do not know and cannot verify from historical records." (Morrison, Turning from Truth: A New Look at the Great Apostasy, p. 5).
Nevertheless, it is Joseph Smith's testimony (and mine, and Mormons' in general) that the Apostasy did happen, over the period of centuries, until the church Christ founded was in error in many ways. The fact that God restored the Priesthood through Joseph Smith testifies of this fact. So I guess what I'm saying is, the testimony that the gospel was restored is the greatest evidence that the Apostasy occurred. You can gain this testimony by reading and studying the Book of Mormon, and asking God with pure faith, with real intent, whether it is true or not. If it is wisdom in Him that you should receive this testimony, and you do your part (study and prayer), God will let you know of the truth of the Book of Mormon's veracity. In this manner, you can know if the Apostasy occurred, by the following logical sequence:
If the Book of Mormon is true, then Joseph Smith was a true prophet.
If Joseph Smith was a true prophet, then the Priesthood he restored is true also.
If it was necessary for the truth to be restored, then the Great Apostasy did happen.
It all comes down to whether you believe in the testimony of Joseph Smith and the truthfulness of the Book of Mormon.