Ancient Letter, Modern Mystery: The “Secret Mark” Translation - Biblical Archaeology Review
The above link at Biblical Archaeological Review contains a letter, supposedly from Clement, discussing the various Secret Books of Mark that came up in his day. Clement was a disciple of Peter. He explains that Peter and Mark (also Peter's disciple) wrote copious notes on the gospel. Mark took these down to Alexandria, where he wrote a secret gospel. Gnostic groups, including the Carpocratians, obtained a copy and adulterated it to fit their own beliefs.
Still, Clement states certain things that are true about the original Secret Book of Mark:
1. There were secrets taught by Jesus to the initiated, things which were "not to be uttered."
2. Only a few were allowed to know the secrets, while most Christians were to only know the basics given in Mark's regular gospel.
3. The secret truths led the initiate "into the innermost sanctuary of that truth hidden by seven veils."
He mentions one story from the Secret Book of Mark, quoting from it, concerning a young rich man who Jesus raised from the dead. Then Jesus taught him the secret things, "And after six days Jesus told him what to do and in the evening the youth comes to him, wearing a linen cloth over his naked body. And he remained with him that night, for Jesus taught him the mystery of the kingdom of God." The Carpocratians and others used this to claim Jesus was having late night romps, but Clement and Mark knew that the secret initiations required the teachings to be done like this. LDS will note how this sounds like their temple initiatory work.
Once again, Joseph Smith "guessed" right on several things:
1. Some important teachings of Jesus and his disciples are not to be found in the Bible.
2. These important teachings included secrets that were to be kept from the world, and even from the regular members of the Christian Church.
3. These ancient teachings (veils, linen cloth, secrets that are unutterable) make sense in a modern LDS temple context.