I am quite comfortable to be judged for teaching my daughter the Truth as best I understand It.
If you believe you have found the truth, and it is that Christianity is false, I can totally understand your wanting to teach this to your daughter. However it makes it totally inappropriate for you to play at Mass.
I have been thinking about this. I think that, given that you would prefer your daughter not be a Christian, it would be wrong for you to join her at Mass. It feels like undermining your wife’s commitment to raise her Catholic. It would also be either a tiny bit dishonest (if you don’t tell the others at the parish that you would prefer your daughter not be a Christian) or undermining for possibly other children in the choir (if you do tell them).
I think that Mass is the one time you should probably not accompany your daughter. I think that the parents of other children in the choir have a right to expect that their children will not be exposed, at Mass (and other Church-related activities, like rehearsals), to non-Christian ideas. And how could you not express non-Christian ideas without lying? And your participation in musical lies (to you) is not a good example to anyone.
If you want to go to Mass (and this would be great!!), you should go to another Mass where you wouldn’t be sending mixed messages to your daughter. And in any case it just doesn’t make sense to have a non-Christian helping to lead the music.
Sorry, it’s just been on my mind. Frankly, my primary concern isn’t what’s best for you, I think the important thing here is what is best for your daughter. I really don’t think you playing for Mass would be best for her. However I can see why you, who must want your daughter to be a non-Christian when she grows up (since you think Christianity is false), may have different ideas about what is best for her. But the fact that your aims for her are completely opposite of the purpose of going to Mass is the reason it would not really be 100% ethical for you to do this.
Of course, I must include the standard caveat: “I may be wrong.”
–Jen