Oh no, I really screwed up when I sent my grieving friend this message ... What do I do now?

Since she's a Christian too, she shouldn't have been offended by that. It's possible that your second message to her is what offended her. Or it's possible that in her grief she's just really easily offended. Just give her some space and she should come around eventually, you said you already apologized, that's about all you can do right now.