Actually Communism IS a religion (as is the modern ideology of secularism, democracy, and free market).
Religion is a collective system of fundamental beliefs, asumptions, values, and behaviors, which organises the group, movement, society or culture.
The religious roots of Communism are well documented - Hegel in his synthesis of late Protestant theology and of the Enlightement made God the Absolute, which achieves self-realisation through the material universe and the historical dialectical progress.
Communists reversed Hegel's doctrine by believing that the material world itself aquires future divinity through the historical struggle. Thus the Communist worships the future mankind as god by joining the party (the church) and by the sacrifice of revolutionary activity. The Puritan dream of late Protestantism, the new world order of XVIIIc. deists, and the Jewish hope for the Mesianic kingdom in this world, are united in Marxism.
In this aspect, Communism is akin to National Socialism (based on race competition), modern sects (based on millenarist interpretation of the Bible) like Jehova Witnesses or some Protestant fundamentalists, the presently dominant social engineering of political correctness combined with the antinomian licence (promotion of pederasty for example), militant secularism (removal of more traditional religious competition), democracy (sacred voice of the people) and the blind trust in the magical power of free market.