See, who says the left isn’t religious? Satanism is historically more ritualistic than a true belief, however (except for the most fanatical I suppose). It represents opposition to the established order, going back to the black masses of the middle ages. And leftism, by comparison, represents a political faction seeking change. The two actually fit well together.
Man really can’t live without some form of religion, and the Left is really only against Christianity, which they will replace with Satanism, or some other false religion, such as the idolatrous worship of their murderous Great Leaders (”cults of personality”). Stalin, Mao, Kim Jong Il—all were deified.