Selected quotes V. I. Lenin - The Attitude of the Workers’ Party to Religion:
"The party of the proletariat demands that the state should declare religion a private matter, but does not regard the fight against the opium of the people, the fight against religious superstitions, etc., as a “private matter”.
The opportunists distort the question to mean that the Social-Democratic Party regards religion as a private matter! "
"This point in the Erfurt Programme has come to be interpreted as meaning that we Social-Democrats, our Party, consider religion to be a private matter, that religion is a private matter for us as Social-Democrats, for us as a party. Without entering into a direct controversy with this opportunist view, Engels in the nineties deemed it necessary to oppose it resolutely in a positive, and not a polemical form. To wit: Engels did this in the form of a statement, which he deliberately underlined, that
Social-Democrats regard religion as a private matter in relation to the state, but not in relation to themselves, not in relation to Marxism, and not in relation to the workers’ party."
"We must know how to combat religion, and in order to do so we must explain the source of faith and religion among the masses in a materialist way. The combating of religion cannot be confined to abstract ideological preaching, and it must not be reduced to such preaching. It must be linked up with the concrete practice of the class movement, which aims at eliminating the social roots of religion. "
"And he must not succumb . . . to the philistinism and opportunism of the petty bourgeois or liberal intellectual, who boggles at the struggle against religion, forgets that this is his duty, reconciles himself to belief in God, and is guided not by the interests of the class struggle but by the petty and mean consideration of offending nobody, repelling nobody and scaring nobody—by the sage rule: “live and let live”, etc., etc. "
"A Marxist must be a materialist, i. e., an enemy of religion, but a dialectical materialist, i. e., one who treats the struggle against religion not in an abstract way, not on the basis of remote, purely theoretical, never varying preaching, but in a concrete way, on the basis of the class struggle which is going on in practice and is educating the masses more and better than anything else could. "
read the whole thing here:
http://www.marxists.org/archive/lenin/works/1909/may/13.htm