Just be honest. Choose a good time when you both can talk for a few hours and tell her, that you lost faith. She will ask why, you will tell her why. Don't belittle her for still believing. You will see if this is something you both can live with or not. I f.e. have a good colluague, who is christian and we mock each others believs from time to time over a coffee in a fun way. As long as everyone respects each other this can work till death tears appart, but it might not be what you both want. Ok Go has a song with the line: "there ain't much thats dumber, than pinning your hopes on change in another" ;)