Science is the atheists' religion for the modern world, like Christianity or Islam, except that it is more superstitious and less refined. Atheists' belief in science requires faith like religion, but atheists doesn't recognize and accept its reliance on faith.
Throughout the course of modern history, science and the scientific method have contributed substantially to the reduced scope of religious claims about the world as well as the scope of religious authority. Science has done far more to both explain the world around us and help us improve our condition than millennia of religion. It's not surprising that some religious believers resent this and among their response is to deny that science is any different from religion.
An important aspect of this tactic is to insist that science doesn't really provide objective knowledge about the world and that science doesn't utilize a consistently reliable or proven method for acquiring knowledge. Instead, science is supposed to be based on guesswork, "theories," and false beliefs which are all inferior to "true" religions, like Christianity, and their revelations from God, like the Bible.
There is a curious contradiction here because people who argue for this myth end up involved in two efforts which should be recognized as contrary: first, they have to denigrate science and argue that it really isn't as good as defenders claims; second, they have to argue that science is actually a type of religion which relies on faith, not unlike their own religion uses faith.
Claiming that science is a religion is simply wrong because science lacks the major characteristics which define religion and differentiate religious belief systems from other belief systems. If we ignore that for a moment, though, it doesn't make much sense to do this in the context of denigrating science because that implies that one's own religion is also inferior. It would be far more preferable to argue that one's own religion is as good as science, and then that science is also a religion.
Why don't religious believers normally do this? The simple reason is that they can't: the advances made by science, the benefits of science, and reliability of science cannot be matched at any level by any religion. Religions have claimed for millennia that they have received special information from gods, but at no point did any of those gods explain how to utilize electricity, how to improve sanitation, the origins of disease, and so forth. Much of this was already well underway even during the earliest stages of modern science — it didn't even require a fully developed scientific method or community for such progress to be made.
To be fair, it can be argued that a certain amount of "faith" exists with how average person accepts what science says. Few people are in a position to confirm the results of modern scientific experiments so they have to accept what others say based on their experience and authority. Unlike with religion, however, anyone can in principle confirm those experiments on their own — and the ability of others to repeat experiments to make sure they are right is one of the things which defines the scientific method.
Moreover, most people can observe the practical impacts of what science says and thus don't need to conduct experiments to confirm that scientists are right. Not everyone is able to understand the theories behind how electricity operates, but everyone is able to witness the obvious and dramatic effects of electricity at work — both good and bad.
Some religious believers might claim the same on behalf of their god, but there are many believers from many religions claiming the same about many different gods. Not all of those gods can exist, so not all of the claimed "effects" can be attributed to real gods. Everyone, however, uses the same electricity and sees the same effects of electricity. There aren't alternative denominations of "energy" with competing claims about what the "real" source of energy is. Thus the claims about gods and their effects do have to be taken on faith, but the claims of science — like for example the science of electricity — don't need to be taken on "faith" in the same way.