Some people (Marxists, for example) are against religion not because they don't like people believing in something (hell, Communism depends on the people believing that it is the best way to live), but because they think that religion is a way to control people and keep them complacent. Opiate of the masses, right?

Their argument is: Religion is a construct, something that must exist for the control of people, not for their benefit. They see religion as a way of limiting people, keeping them in constant fear/awe/whatever of their God, preventing them from doing very much that is revolutionary.