You don't have to subscribe to their set of beliefs. They are not telepaths, they don't know whether you actually believe or not.
For that matter, it's debatable whether or not the believers actually believe things. They probably wonder themselves sometimes. They don't often make a big deal out of it, if they do find themselves no longer believing... and there is a conceptual framework built into the religion (that of "loss of faith").
Like the ancient religions, being willing to perform the rituals non-cynically is really the only requirement. You and everyone else already perform dozens of non-religious rituals and think nothing of it. What's a few more?
I mean, don't do it if you don't want to. But don't pretend the option's not open to you. Even if there's a clever believer there that somehow reads your mind, you're not unwelcome at most of these places even then.