An old friend of mine is excited to go on a mission trip and asked me for donations. I want to tell her I'm no longer religious (nor do I want to donate). How do I reject her request and break the news?

We haven't talked in a long time but we used to be good friends when we went to the same church. She messaged me just now telling me she's going on a mission trip. She asked if I wanted to read her support letter. I said sure. I didn't even know what that meant.

Well, lo 'n' behold, a letter with pretty graphics addressed to me with the first paragraph about our wonderful sisterhood in Christ and me being a role model. Then 3 paragraphs about the mission. Then a paragraph asking for monetary support.

I don't just want to make some lame excuse about how I'm going to unfortunately be unable to provide the financial support but I will commit to the prayers blah blah blah. I want to be honest with her. I've been honest with mutual "sisters in Christ" but I guess word didn't travel.

So I'm going to be a party pooper, but how should I do it? I didn't loudly proclaim my leaving Christianity but I want to tell every Christian friend if the topic comes up that I'm no longer Christian.

Ok this seems kind of trivial now, but I'd like to hear from you guys.

Edit: Thanks for the responses. I guess I didn't make it clear that it's very important to me to tell her that I'm no longer Christian. I want to tell all my Christian friends (and also non-Christian friends who knew I was Christian) when the topic comes up. I'm just going to tell her that, and she'll realize I'm not donating.