Marx on Religion as the opium of the people

Religion is the sigh of the oppressed creature, the heart of a heartless world, just as it is the spirit of a spiritless situation. It is the opium of the people. The abolition of religion as the illusory happiness of the people is required for their real happiness.

(Karl Marx, Critique of Hegel's Philosophy of Right)

It is interesting now living in an age where we have been for the most part freed of the trappings of "religion", where religious believers are demonized as ignorant bigots, yet there seems to be so little true happiness. I wonder how Marx thinks that, once freed from religion, there can be any happiness in a "heartless" world?