What is the difference between communism and christianity?

They typically have nothing to do with each other. Communism is an economic system of centralized planning (without computers), little or no private property, anti-unions, single political party (aka Leninism), and tends to suppress any popular uprisings. Communism in effect really no longer exists although there are some Americans who believe still exists as an alternative to their blend of capitalism and socialism (which some believe should be solely capitalist). Communism probably exists as a economic system in less than five small countries in the world.