Here we have the first mention of any priest in the Bible. Melchizedek was a shadow of the things to come and the only priest with an endless priesthood at the time. The next time we see this would be Jesus Christ! No where in the Bible does it say that Adam ever held a priesthood or was actually Michael the Archangel! God’s priesthood of Aaron’s sacrifices was set up at the time of Moses and Aaron. Read the account of it from Exodus chapter 40.
“The Priesthood was first given to Adam; he obtained the First Presidency, and held the keys of it from generation to generation. He obtained it in the Creation, before the world was formed, as in Gen. 1:26, 27, 28. He had dominion given him over every living creature. He is Michael the Archangel, spoken of in the Scriptures. Then to Noah, who is Gabriel; he stands next in authority to Adam in the Priesthood; he was called of God to this office, and was the father of all living in his day, and to him was given the dominion. These men held keys first on earth, and then in heaven.” Joseph Smith, History of The Church of Jesus Christ of Latter-day Saints, 7 vols., introduction and notes by B. H. Roberts [Salt Lake City: The Church of Jesus Christ of Latter-day Saints, 1932-1951], 3: 385.