I considered the offense that had already happened by changing the picture settings so that she couldn't see it, as she requested. Before then I hadn't actually realised that she was a Christian.
People make light of things that important to me all the time; I know it's just teasing. My ex used to make fun of me being a music student all the time (and no, that is not why we broke up).
I'm still talking to the person now; we've agreed to disagree, and we are now discussing various musical activities that we've been involved in.
*edit: oh, and by NOT being sarcastic at her.