Just sit down and speak your mind and tell her your true feelings.

I felt the urge to admit to my parents that I was an atheist while they were both christians. I was "compelled", by our holy savior, if you will , to tell them about it. It nearly drove me insane, and I eventually "came out" with it. It shocked them, and I believe that if I say that your wife will not be shocked, from my experience, it would be a lie. However, in the end, I am extremely glad that I told them my true beliefs. Best of luck to you, either path you choose!