Karl Marx
Karl Marx (1818-1883) was a German economist who sought to set socialism and communism on, what he saw as, a scientific basis. Marx was born in Prussia, educated in Germany, and a writer in London.
Marx saw "primitive" socialist thought as fundamentally a moral movement with no base in a practical science. Marx's solution was to ground an analysis of society on his principle of dialectical materialism.
Dialectic was a principle created by German philosopher Georg Wilhelm Friedrich Hegel which, although ferociously complex in his works, is based on the fairly straightforward conception of action and reaction. A situation arises that is less than perfect ("thesis") which provokes a reaction ("antithesis"), eventually a settlement is reached ("synthesis") which, is still far from perfect and provokes its own antithesis. Hegel saw the driving force of changes being the "Geist", variously translated as "spirit" or "mind". Marx took the dialectic and replaced Geist with materialism, at root the metabolism of man. To put it bluntly: you've got to eat!.
The necessity for material sustenance gives rise to an imperfect means of production, which changes as the internal contradictions of thesis and antithesis work themselves out. A key feature of Marx's sociology was that the competition for resources splits society into mutually antagonistic classes ("The history of all hitherto existing society is the history of class struggles.") whilst the operation of the dialectic serves to continually reduce the numbers of classes until, in the industrial age, society was almost wholly split between "bourgeoisie" and "proletariat".
"Das Kapital" ("Capital") seeks to demonstrate that the bourgeoisie make profits by exploitation of the proletariat. The exploitation arises where the value of the goods produced by workers is much more than the wages paid, and this "surplus value" is literally stolen from workers by owners. Marx sought to show that the competition amongst the bourgeoisie forced them to exploit their workers as much as possible. If an individual bourgeois refused to continue the exploitation he would soon be forced into bankruptcy or be taken over by someone who would. The result is a relentless increase in the misery of the proletariat who, eventually, would rebel and replace the existing capitalist system with a socialist/communist system.
Marx is considered very controversial in religious circles for his statement, "Religion...is the opium of the people." This was written in his 1843 work "A Contribution to the Critique of Hegel's Philosophy of Right" and expresses Marx's belief that religion offers a false palliative to assuage the suffering of the oppressed, which he felt prevents people from seeking to cure the conditions which cause that suffering. It is often cited as evidence of Marx's atheism and generally reflects Marx's disdain for those who have found belief in powers higher than man himself.