Well, I'd say if you're actually anti-Christian, it would be a bit unethical. Plus, after a bit of time you'd probably be uncomfy and they'd sense you were annoyed with furthering Christian ideology.
BUT, if you are just not Christian, but have an understanding of the religion and empathy (see Wil's definition) for Christians... then that fits the bill. They don't ask you to *be* Christian, just to be understanding and empathetic. So I'd just be honest- that you participate in interfaith, you would love to design for them, you're understanding of the religion of Christianity, etc. Stick to what is real.
My 2 cents.
Love some of the images, BTW- especially the ones focused on light/darkness.