If you are going to feel offended by her belief, then you're going to have a problem. You have to expect her to be offended by your lack of belief, and be prepared to deal with it. When you got married, you were a believer. Whether it's fair or not, she expected you to be a believer until you died, so on this matter you owe her a little more consideration than she owes you. She is not only going to feel offended, she is going to feel very vulnerable. Make sure she knows that you want to stay together. Be as honest as you can, as kind as you can, and assure her that your feelings about her have not changed. Use lots of non-verbal communication. LOTS.
Mark has given some good words of wisdom. The only thing I can add is this. Imagine, for a moment, when you were a believer (most of us on here, myself included, at one time or another were). Now, as a believer, also imagine that someone you dearly loved came out to you and told you what you believed was nonsense. How would you feel? More importantly, how (now that you're a non-believer) would you have wanted them to 1) tell the truth about their lack of belief, and 2) notwithstanding that lack of belief, expressed to you how they stilled loved and cared about you.
Once you've figured that out - and I'll admit, it's damned difficult - I think you got your answer.
If you don't have kids already and are thinking about doing so, I think it is critical that you tell her now. She needs to know. I place high odds that you will both undermine each other in this one aspect of raising children. If you already have children the issue is much more complicated. I would recommend not directly contradicting your spouse on anything religious with the exception of new earth creationism. At the same time, I would encourage a strong interest in the sciences and scientific method.
Remember this is just an opinion. I waited until I was 29 to find the right heathen woman to marry. Actually it took that long to find someone gullible enough to marry me. So, I am not going to know that much about your situation.
We have two sons. As I read these responses, I felt a great sense of relief. I really don't see my wife reacting in a marriage-ending way (She's a divorced, pro-choice Catholic who believes Adam and Eve and Noah's Ark are myths, strongly believes in gay rights and gay marriage, etc).
Naturally, I agree that I need to be the patient one here and give her time to accept that I've come to this conclusion. I don't think the sky will fall on our marriage. I was critical of Catholicism before we got married, and she was okay with that. So really, not much changed with respect to my view of her religion.
Still, I can't pretend this won't be a big deal. I'm going to take this slowly and carefully. One day she'll ask why I do not insist on going to my old evangelical church anymore, and I'll tell her step by step: I, too, am pro-choice. I, too, believe the Creation story and Noah's Ark are myths. Etc.
Thanks for the advice. I'd love to hear more.
I think slowly and carefully is a good approach. Maybe you could discuss topics first, so your stance won't be a shock. Hopefully she won't be defensive when discussing topics before you finally tell her. I think most people think logically before they feel cornered. Then they will fight you no matter what.
Are you certain of her beliefs? My wife and I kind of came around at about the same time, not that either of us was ever really religious. Her only comments ever were "is there something wrong with me that I don't believe like these other people do?"
I just wanted, again, to thank everyone here for the support and sound advice. My wife and I still haven't had the talk, and I have no intention of rushing it. We had a delightful Christmas and our boys sang happy birthday to baby Jesus. They also were very excited about Santa Claus visiting on Christmas. We'll have our talk, or series of talks, when the time is right.
Glad to hear that you are having a good holiday. I wish you luck when the time comes.