In a 1985 speech to BYU students, Bruce R. McConkie related a doctrine which is one of the most distinct of classic Mormonism, but I wonder how common it is nowadays. According to McConkie, all of the ancient Biblical prophets were actually Mormons who were baptized and practiced celestial marriage:
Besides the historical claims about the Lehites and other Book of Mormon peoples, this belief is probably one of the most unique to Mormonism. It's also essential to its claim to be the "restored truth" from the ancient times.Christ is the God of our Fathers. He is the God of Adam, and Enoch, and all of the saints who were before the flood. He is the God of Abraham, Isaac, and Israel, and of all the holy prophets. As Jacob the Nephite said,
Thus all of the ancient saints—all those from Adam to Noah, and from Noah to Abraham, and from Abraham to Moses, and from Moses to the coming of Jesus in the flesh—all of the truly faithful ones of old had the gospel. They were baptized and received the gift of the Holy Ghost; they were endowed with power from on high; they received the blessings of celestial marriage. The gospel is everlasting, and all men in all ages are saved by obedience to the same laws and the same ordinances.We knew of Christ, and we had a hope of his glory many hundred years before his coming; and not only we ourselves had a hope of his glory, but also all the holy prophets which were before us.
Behold, they believed in Christ and worshiped the Father in his name, and also we worship the Father in his name. [Jacob 4:4–5]
I don't have much contact with Mormonism nowadays, so I'm wondering if this doctrine has faded into the background given that one can easily see what ancient Jews believed and that while there is plenty of evidence for Hebrew paganism, there is not any evidence for Christian beliefs, let alone Mormon ones.