I know you probably went about this in a different way than I did, but on the other hand, your kid is not Maeve, right? I mean to say, please try not to take offense at the following essay, even if it is not how you would have gone about it. Religion is a tricky subject with children, doubly so with my scientific little bean.
Nevertheless, I broached the subject of religion with Maeve yesterday, in the kitchen while I put clean dishes away. I explained to her that a 'religion' is a set of beliefs and customs that bring people together, a set of beliefs that are outside the realm of proof.
Of course, Maeve dismissed these proofless ideas at first, and told me it was all pretend. I explained again that there are a lot of ideas in the world that exist without proof, yet people believe them every day, including me. I told her I believe that people are inherently good, despite the fact that I can't prove that. Then I gave her a quick overview of some world religions, especially the ones in which members in our family believe, including my own Unitarian Universalism.
She looked thoughtful for a minute. Then she said, "Mom, I have a religion."
I said, "Oh yeah, Sweetie? Tell me about your religion."
"I believe all people have a good heart," she reported earnestly.
And I kissed her cheek and told her that was a very good religion.