Karl Heinrich Marx (1818-1883) was a revolutionary activist and critic of capitalism. He published much over the course of his lifetime, from his early Communist manifesto, presenting history as a struggle with classes and ending with "Workers of all land, unite!", to his later Das Kapital, a lengthy work in which he attempted to prove that capitalism is fundamentally self-destructive.
In his lifetime, he acquired numerous followers, some of whose beliefs once provoked him to say that "if that is Marxism, then I am not a Marxist". And Vladimir Lenin and other Communist revolutionaries have gone even farther with his ideas.
1844- Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people. -- Karl Marx, A Contribution to the Critique of Hegel's Philosophy of Right, Preface
For full texts by Marx (and Engels) on Religion visit the Marxist Internet Archive