In the Doctrine and Covenants it states that Michael the Archangel is Adam. Is there any evidence from early Christian or Judaic beliefs that confirm this was at least a theory or is this for the most part new information?
What we know of Adam, Michael (the Ancient of Days), has been revealed through modern revelation, at least in our generation. We are informed in our Bible Dictionary,
“Adam is the Ancient of Days and is also known as Michael. He is the archangel and will come again to the earth in power and glory as the patriarch of the human family preparatory to the Second Coming of Jesus Christ.”
The scripture verse in our Doctrine and Covenants I believe you are referencing would be, “And the Lord appeared unto them, and they rose up and blessed Adam, and called him Michael, the prince, the archangel.” (D&C 107:54)
Joseph Smith declared,
“Daniel in his seventh chapter speaks of the Ancient of Days; he means the oldest man, our Father Adam, Michael; he will call his children together and hold a council with them to prepare theme for the coming of the Son of Man [see Daniel 7:9-14]. He (Adam) is the father of the human family, and presides over the spirits of all men, and all that have had the keys must stand before him in this grand council.”
Early Christianity and Judaic history doesn’t appear to have anything referencing Adam as Michael, at least from what records I am able to search and read, and then again, I am no expert in historic teachings from Judaic or Christian ministers.
If the Lord revealed to Joseph Smith that Adam was/is Michael, then it wouldn’t be hard to accept that Adam probably knew who he was also. If so, he would have taught his children, and they would have taught their children (at least the righteous posterity), and thus passed down. Abraham is known to have seen many things and it wouldn’t surprise me if Abraham received the same inspiration as Joseph. This would have been the same for Isaac and Jacob. Each of these fathers were in intimate communication with Jehovah. In light of these thoughts, I would say, this isn’t new information which has been revealed, but lost doctrine which has been restored in our dispensation.