Who was Adam in the pre-existence?  

Doctrines of Salvation 1:90; “The first man placed upon the earth was a perfect being, a son of God. He was Michael, the Archangel, who had reached great distinction and power before he ever came to this earth; and who helped to frame this earth while he was yet a spirit, just as our Lord and Savior Jesus Christ was a spirit before he was born in Bethlehem.  Michael, after being placed upon this earth, is known as Adam.”

Genesis 2:7; “And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.”

Genesis 3:15; “And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.”

Genesis chapter three shows us what constitutes the fall of man.  Adam who had been given dominion over the plant and animal world wasn’t content with the power and exalted status God had bestowed upon him.  Believing a lie from Satan, Adam and Eve got it into their heads they could also hold the exalted status of being like God. This one selfish act put enmity (hostility, hatred, ill will, animosity) between God and man. Adam was not a living soul until God created him from the earth and then breathed life into him. 

Likewise, the Mormons took their already exalted status God had given to mankind and stretched it into another dimension Adam and Eve hadn’t thought of. Not only do Mormons believe they can think like God and become a god themselves, they also believe they existed with God before coming here to earth, in other words they are “co-eternal” with God (Teachings of the Prophet Joseph Smith, pg. 353).

They don’t believe God has the omniscient and omnipotent wherewithal to bring them into existence out of nothing, which is exactly what He did.