For example, unless the aliens are very much "in our image" their god will not look like the Christian god.
But what, exactly, does the Christian God look like? I was raised Christian (Catholic specifically), and I was taught that God is immaterial. The bearded bloke in the sky is just a traditional pictorial representation and nothing to do with the reality, and the 'in our image' is speaking in a metaphorical, spiritual sense.
An alien race who worshipped a creator represented as having purple tentacles would only pose a problem for a very theologically-immature Christianity, not the one any Christians I know believe in.
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