I spoke to a Jew once and he told me it is impossible for God to ever be a man! People think they can just limit God to what seems godly to them, ironic because we are ungodly.
Well all I can say is that God can do much better things than the devil and a man is a better thing than a snake.
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When people say God can't become a man, you can always challenge them with the words of Jesus. What man can speak his words and not be God?
When I read about "the serpent" I tend to think this was a figure of speech. Like a title of something. Logically, Eve would have deceived much easier if Satan appeared as himself. (an angel) Had he appeared as a literal serpent, Eve may have recognized the trick. Remember, Adam and Eve were sinless and very keen to spiritual things, compared to their descendents.