A theory is a belief, even if scientific. A scientific theory says, given all the known facts and observations, we believe this explanation to be true. Yes, it carries much more weight and credence than "I believe in something because a 6000 year-old book told me to," but it's still a "best guess" as to the true explanation of the facts.
Like it or not, we never really have a full picture of complex systems, like astrophysics, quantum mechanics, biological systems, and many other examples. The best we can usually hope for is a best guess of the explanation, based on observation, so that we can build on that knowledge and move forward. That best guess is also a belief, and from time to time, that belief is re-evaluated and either confirmed or invalidated for a new set of best guesses. That process and transformation doesn't make it *not* a belief, it just makes it one that we're not scared to change from time to time, unlike an absolute belief in a deity.