Michael, in the chronology of the story Satan, represented by the term "crafty beast" wasn't something that resembled a snake until after he successfully tripped up Eve. But even then its a poetic term for the rebel celestial. The point is, it would be more accurate to say sin entered the world through Satan then Adam.....but the world had already fallen, death is normal for man but it wasn't for Adam and Eve, 2 incarnate celestials who could use "the tree of life" to sustain life indefinitely as long as they remained loyal to the Father.
I believe, as is revealed in the UB, that there is a much bigger, older story with much more explanation. The "crafty beast" or fallen Prince of this world, had been on the evolved earth 500,000 years before Adam and Eves arrival. He had a staff of 100 super-mortal volunteers who came with him to begin the teaching about the Universal Father as well as provide spiritual guidance and leadership for the populated earth. Lucifer, a high administrator who oversaw many inhabited worlds under the creatorship of Michael, fell into sin and rebellion against the rule of the unseen father on Paradise. He managed to convince many other planetary princes to follow him as well as many different orders of celestial beings. "There was war in heaven", an ideological war of allegiance between the rule of the unseen the Father through his Son Michael, or to Lucifer, who sought to set up his own administration.
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