Truth is, you can't.
You can attempt to give a witty, faith jostling retort, but I don't think that would have the impact you're seeking to have.
Rather, you can outline (briefly or not, it's up to you) why you decided to ditch your faith; the moments that grew to culminate in you making that leap. I don't see any need to be antagonistic about it, almost like he is in that letter. Rather, outline the above, and conclude that you (personally) don't need a god, or a religious belief, to be who you are. Your love and kindness doesn't require an un-evidenced big bearded man in the sky.
You can attempt to give a witty, faith jostling retort, but I don't think that would have the impact you're seeking to have.
Rather, you can outline (briefly or not, it's up to you) why you decided to ditch your faith; the moments that grew to culminate in you making that leap. I don't see any need to be antagonistic about it, almost like he is in that letter. Rather, outline the above, and conclude that you (personally) don't need a god, or a religious belief, to be who you are. Your love and kindness doesn't require an un-evidenced big bearded man in the sky.
![[Image: 146748944129044_zpsomrzyn3d.gif]](https://images.weserv.nl/?url=i629.photobucket.com%2Falbums%2Fuu13%2FRXM843%2F146748944129044_zpsomrzyn3d.gif)
