If Mary knew Jesus was God, then she ALREADY knew that 'all things were possible with God'. Therefore, she 'knew' that Jesus could walk on water, cure a blind man, turn water into wine, etc. She 'knew' that he could call down a legion of angels at a moments notice. She knew. She is not God, but her abilities and prowess are far beyond anything any human can conjure up.
Francis, for what it's worth, I'm pretty sure that neither TimTheEnchanter nor the blogger to whom he links is intending to defend the lyrics of the song.
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