These words of Marx and Engels (Communist Manifesto, 3:I:a) suggest that Christianity may be initially quite useful for the socialist cause:
Nothing is easier than to give Christian asceticism a socialist tinge. Has not Christianity declaimed against private property, against marriage, against the State? Has it not preached in place of these, charity and poverty, celibacy and mortification of the flesh, monastic life and Mother Church? Christian socialism is but the holy water with which the priest consecrates the heart-burnings of the aristocrat.
Even though the abolition of all religion is ultimately a necessity in their suggested system, Marx and Engels recognize the practicality of incremental progress toward their goals. Hence, they welcome a slight retasking of Christian ideas in order to facilitate societal drift toward socialism, and ultimately toward communism.
Marx and Engels perceive that all conflict is traceable to class struggle, and that class struggle is economic at its core. Redistribution of wealth is a key to neutralizing the conflict and liberating the oppressed from their oppressors. For Marx and Engels, class struggle is an economic issue that can only be resolved politically, hence their political efforts toward an economic final solution. Capitalism represents, for them, a system that enables continuing oppression of the proletariat by the bourgeoisie. Capitalism is a great enemy to be conquered, and one that will naturally fall—if and when the proletariat realizes their great strength, unites, and acts in accordance with Marx’s and Engels’ designs. Read more about What's Wrong with Capitalism?