outofthelionsden - Your friend obviously cares about you and IMO deserves a reply. A lot of good advice has been given already- one way that a note that might be helpful could be: (If true in your case of course)
"Thank you for reaching out to me and communicating, I appreciate it. I have not left God, and have not abondoned my faith in him and his Son. So many have shunned me without ever hearing what happened, choosing to believe rumors instead. If you want to talk about it with an open mind, then I would be happy to share what I've learned with you.
Regardless, I value our friendship and my door is always open.
With Kind Regards,
[your name]"