"Man is a Religious Animal. He is the only Religious Animal. ..." — Mark Twain quote meaning

Man is a Religious Animal. He is the only Religious Animal. He is the only animal that has the True Religion--several of them. He is the only animal that loves his neighbor as himself and cuts his throat if his theology isn't straight. He has made a graveyard of the globe in trying his honest best to smooth his brother's path to happiness and heaven....The higher animals have no religion. And we are told that they are going to be left out in the Hereafter. I wonder why? It seems questionable taste.