“Surplus exploitation” is Marx’s term for the value created by the worker that is not accounted for in the worker’s wages but instead accumulated by the capitalist. Capitalism requires a continuous expansion of surplus (regardless of limits on resources) accumulated by a select few, resulting in wealth disparities and class antagonism.
An example of this is the empire of Rupert Murdoch who owns the companies that produce Fox News as well as shows such as “Family Guy,” a cartoon that makes fun of the same type of narrow-mindedness seen in pundits of the aforementioned network. Far from being a contradiction, this is the capitalist ethic at its purest: the creation of profits is ultimately the only value a capitalist system is capable of caring about and exploiting. Marx believes everything in society (even religion) can be analyzed according to how it relates to class antagonisms created by exploitation.
Marx’s famous equation of religion as an opium of the masses does not mean that he is altogether antagonistic toward religion. Marx himself used opium, a common medicine in mid-19th century Europe. For Marx, religion and opium are painkillers that are not only addictive but even worse, since they treat symptoms rather than diseases. Religion is both an expression of and protest against real suffering, a metaphysical and ideological surplus exploitation.
Marx believes the need for religion will dissipate as class antagonisms caused by the capitalist condition (surplus exploitation) is properly dealt with. Later, Marx’s partner Engels came to recognize the end of capitalism in revolution would not necessarily kill all forms of religion–he believed there was in Christianity a revolutionary potential that could persist and continue to aid political liberation.
Marx is formed by Hegel’s concept of the dialectic. A particular state of things cannot feel the need to change until it is faced with a contrasting state. The proper dialectical solution does not simply synthesize the two contrasting states; instead, the dialectical solution incorporates the experience of the the internal antagonisms of the current state of being and sublates into a higher mode of being. This is important to understand Marx’s dialectical materialism. For a society to move forward (sublate) into a more liberated political order, the antagonism of the class struggle must be felt clearly.
Christianity can, at its worst, bypass this antagonism by mitigating the proletariat’s frustration with their impoverished existence. At its best, Christianity can provide a narrative that 1) embraces material reality in the incarnation and, 2) after the death of God, leaves only the Holy Spirit as the remnant among a community that desires to bring liberation. For as Marx writes at the end of his Thesis on Feuerbach, “The philosophers [and theologians] have only interpreted the world, in various ways; the point is to change it.”