Science is not a “belief system” but a process and methodology for seeking an objective reality.
I agree that science is a set of principles relating to epistemology, but I think it's also clear that we use the word in multiple ways which go beyond this. So I might say that science is all of these:
- An epistemology. By which I mean that "science" is the word we use to describe a particular philosophy about what distinguishes knowledge from non-knowledge, and what kinds of justifications can reasonably legitimate propositions about the world in order to say that we "know" those propositions are true (i.e. empiricism)
- A diverse collection of methodologies and methods applied to different domains. What makes methods and methodologies "scientific" is the attempt to make them harmonious with the epistemological basis of the philosophy of science.
- A somewhat vague way of referring to a set of social institutions, including universities, research organizations (like CERN), government agencies (NASA), and professional organizations (like the APA or the AMA)
- An even vaguer way of referring to some modern secular worldviews for which scientific epistemology and institutions are central, as well as the body of knowledge associated with them.
Taken altogether, I think one can understand what kind of comparison a person is making when they describe a certain scientific worldview as "religious", although I also agree it's too reductive, typically too pejorative, and isn't always completely clear. But if you think about traditional religious belief and practice incorporating both social institutions and and beliefs about the world, along with some epistemological principles which are thought to legitimize both of those, then you can see how scientific epistemology also serves to legitimize various scientific social institutions and beliefs about the world, along with some forms of ethics.
Additionally, in practice we place a certain amount of trust in those institutions and processes without being able to dutifully and individually apply NDT's principles in every case, because no one has enough time to do so on every possible scientific topic. It's not surprising to me that people compare that trust with the trust others have in traditional religious institutions or organizations.
It seems to me that when religious folks attack atheists as treating science as a religion the mistake they are making is not so much that there is no comparison, but in thinking that because there's a comparison that there is also some exact equivalence in epistemological validity or trustworthiness. So my response is not "science isn't religion!" but more like pointing out that a certain scientific ethos
does serve a similar role (in part) to traditional religion, but opposition to some religious organizations/institutions doesn't constitute a rejection of the need for something to serve that role (i.e. we're not treating science and religion hypocritically), and within that specific context science has a lot of advantages which justify the trust we place in it.