People and churches are funny things. Some people feel like they're not doing their duty to God if they don't try to get everyone they know in the same boat with them. Telling her what you think of her and her church-going ways ISN'T the way to handle this with grace and civility. Simply send her a card back, saying that you are flattered that she would invite you, but that you are not currently interested in this kind of social activity. Be polite, precise, and pleasing. Some church-going people simply will not take no for an answer to their well-meant invitations to church, so expect her to try and invite you to church again. However, in this case, you just want to tell her nicely and simply that you aren't interested in seeing the play, but that in no way is a reflection on her.
If all else fails, tell her you think that frivilous enteretainment is Satan's foothold in the world, and you refuse to have anything more to do with her or her church for suggesting it in the first place.
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Sage knows our mythic history, King Arthur's and Sir Caradoc's
She answers hard acrostics, has a pretty taste for paradox
She quotes in elegiacs all the crimes of Heliogabalus
In conics she can floor peculiarities parabolous
-C'hi