This girl keeps on trying to accept God into my life or something, how do I decline without offending her?

[Underling]

So, I'm kinda new to my high school, and a couple days ago in homeroom I was talking to my friend about my dad (who recently got diagnosed with cancer but that's really beside the point) and the conversation eventually swung around to religion, and I told my friend that I was an atheist and I believed in observations of the natural world instead of faith/God.

Well, this really nice girl who sits behind me must've overheard, because later that day she wrote on my Facebook:
"Hi Cat, I know we don't really talk or hang out, but what you were saying in homeroom really affected me. I know you're going through a really hard time right now and you feel like God has abandoned you, but trust me he has NOT and he loves you, and if you turn to him, everything's gonna be all right, I'm praying for you, I hope you let God into your life, I think you should come to my church's Teen Club" and a whole bunch of other stuff like that. And then she invited me to sit with her at lunch, but the swung the convo around to religion and God.

I want to tell her that I don't believe in God and I politely decline, butI'm afraid I'll offend her, which of course I don't want to do. She's really sweet and I really appreciate her concern. What should I say?
Thanks in advance...