I've never been as close to anyone as I am to my husband. But he doesn't know I'm an atheist. I think he knows I'm agnostic. He is a believer. Is it important to tell him? Why can't I? I'm not ashamed of my beliefs.
Will it affect the way he views you at all? Ifhat special, lovely kind of believer who respects other people's own opinions, then it's probably best to be honest - as it usually is in any relationship.
If you think there's any chance it might change how he feels about you, I'd keep it quiet - you love one another, and there's no point in risking that for something which, in the big scheme of things, isn't especially important.