One can be a purist about it and say that believing in something that is not true, even if it makes you happy, is a bad thing.

But that denies the truth of the matter, which is that religion once provided an anchor in a vast, frightening and inexplicable world. But that world has shrunk, has become tame, and is now understandable. So religion has mutated into a system designed to persuade the poor to accept their poverty, and the oppressed to forgo freedom, in this life on the promise of joy and comfort in the next. It is one of the ways in which the poor can be made to work while acepting that they will never see the benfits of that work in their own lives.