Related Posts: The Fall of Man: Part I; Adam-ondi-Ahman and Eden; Angels
Anti-Mormon writers use a theory called the Adam-God Theory, with which they make the claim that Mormons believe Adam is God the Father; and also, that within the Mormon faith, this is (or was) a deeply held and secret belief.
The theory stems from a sermon delivered by Brigham Young on April 9, 1852. Here is the passage in question.
Now hear it, O inhabitants of the earth, Jew and Gentile, Saint and sinner! When our father Adam came into the garden of Eden, he came into it with a celestial body, and brought Eve, one of his wives, with him. He helped to make and organize this world. He is Michael, the Archangel, the Ancient of Days! about whom holy men have written and spoken–He is our Father and our God, and the only God with whom we have to do. Every man upon the earth, professing Christians or non-professing, must hear it, and will know it sooner or later.
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…Jesus, our elder brother, was begotten in the flesh by the same character that was in the garden of Eden, and who is our Father in Heaven. (JD 1:50-51)