What Is Religion?
By the way, although it sometimes drives people a little nutty, the way I use "religion" is hardly unique to me. The great anthropologist Clifford Geertz defined religion as "(1) a system of symbols (2) which acts to establish powerful, pervasive and long-lasting moods and motivations in men (3) by formulating conceptions of a general order of existence and (4) clothing these conceptions with such an aura of factuality that (5) the moods and motivations seem uniquely realistic."
Thus, it is not quite right to say "scientific materialism is a religion." Some people may be scientific materialists in a offhand sort of way, and find their life motivation in some other fashion. What is more correct is to say that "scientific materialism functions as a religion for many people." In particular, when we find someone claiming something like "We must face up to the fact that our lives are the product of random chance, and that we are alone in an alien and uncaring universe," we are in the presence of a religious belief. Note that even if it is true that "everything is made up of material particles alone," there is no way to derive anything from that statement about what we "must" do! One could as easily decide, "Everything is made up of material particles alone, but we must do everything we can to act as if that is not so!" And think about Geertz's fifth characteristic, and how stunned many scientific materialists are when anyone cannot see that their formulations are "uniquely realistic"!
Similarly, it is easy to grasp how Marxism, or progressivism, can be religions in Geertz's sense. And that sense is not arbitrary: it arose from the work of a great anthropologist exploring the role of religion in different cultures.
And lastly: if you haven't read Geertz, you should!