If aliens invaded, they could use religion against humanity to deceive many, such as disguising themselves as the christian God/Jesus and the angels. They could enslave humanity with little effort on their side while their human followers did most of their work for them, and wipe out all the troublemakers who refuse to accept "Him", leaving their most devout worshippers as their human representatives to manage the daily affairs of keeping slaves in their place.
And those "chosen" humans would undoubtedly feel as if they were part of something wonderful, "doing the Lord's work", as they betrayed and condemned their fellow man to a brutal and pitiful fate.
"I can conceive of nothing in religion, science, or philosophy, that is anything more than the proper thing to wear, for a while." ~ Charles Fort