it would help to answer this Q. by looking at the lyrics:
the song in my opinion clearly isn't just antithetical to 'religion' but to the idea of an afterlife at all, at least not one in which men are relegated to one or the other of two fates (at this point iirc Lennon was a bit of a hindu/buddhist/humanist mix - call it 'new age' i guess, believing in a fatalistic transcendentalism rather than any sort of deity & regarding 'deity' itself as a poorly defined notion that should actually refer to mankind itself in an also poorly defined state of 'enlightenment')
the idea behind this song is that if you removed all religious & spiritual aspects of the human condition, a more 'utilitarian' ethos might emerge, in which we all lived peaceably, shared all possessions and wealth & helped each other out without any prejudice.
the problem is that mankind doesn't become selfless when you remove all religious & spiritual impetus. quite the opposite - men become egoists, even if its hidden beneath a veneer of faux-goodness; in Lennon's construct those good works are really self-serving too, since they are done for the sake of one's own enlightenment, not at heart for the well-being of others.