We are not remotely religious but Christianity is all around us and I decided it would be good for my boys to have at least a nodding acquaintance with God and Jesus. This was one of my projects for December.
The Christian god was hard to introduce. I couldn't find any children's books that presented God in an age-appropriate story kind of way. The books that had stories were super violent - rivers of blood, dead children. The books that weren't violent assumed the reader thought God was real and didn't give any information - "God made me, God loves me" doesn't explain who God is to someone who doesn't know.
So I decided to introduce some friendlier gods, and we got books of African and Greek mythology and read those instead. We talked about how people long ago didn't know much science and told stories of gods to explain the things they didn't know. After a few of these books A concluded that a god was somewhere between a superhero and a magician. I couldn't think of any reason this definition was wrong, so we went with it.
For Jesus, I found a book that focused on his birth. D got a little fixated on King Herod and it became more a story about a king who didn't want anyone else to be king. I told him King Herod was real but he didn't really have all those babies killed (I researched this to be sure). He eventually asked what Jesus did when he grew up and I told him he was a carpenter and a teacher who told everyone to be nice to each other.
I told the boys about the original December celebration of the solstice and the origin of the Christmas tree. I told them how later people chose December as the time to celebrate the birth of Jesus and that for some of our friends and neighbors, Christmas is about Jesus.
Now D likes spotting manger decorations when we're out walking the dog and yelling, "Jesus!" at the top of his lungs.
Interestingly, none of our conversations about God or Jesus triggered any kind of memory of church or teachings from their early life.