DD was very curious about God and faith issues right around the same time. I gave her many analogies about God that fit our belief system. One of the analogies was about the wind and how you can't really see the wind, but you can see the effects of it. Same way God is not someone we see but we can see many miracles, people being kind to each other, beautiful nature etc. I also told her that asking these questions is ok and can strengthen a person's faith. Just speak from your heart and your son will appreciate it that much more.
Jeni
