I agree with Don on the artistic imagery of religious icons, people visit middle age cathedrals and wonder at the art with little regard for the religious significance. Don't get me wrong, I like the idea of the religous deck but if you really want to get a reach to a larger and not necessarily Christian based audience, go the 'awe of wonder and magnificence' route. Now what I would like to see is a faith based Tarot deck, try explaining that to the kids.
Actually, I was thinking similarly but different.
There's absolutely zero need to reach an audience larger than the Christian community. His target audience, bluntly, are believers in Christ. He may get a few people of other (or no) faith, but that's his target. Let that concept settle and there's a lot of things he can do with it.
A faith-based tarot deck is easier than you may realize. The tarot deck originally wasn't used for cartomancy, but for a game by the same name that's still played today in France and some other parts of Europe. The cards don't look like a traditional tarot deck's cards, but there's the exact same number of cards, right down to the suits and "trumps" (you'd think of them as the Major Arcana). The suits are the Anglo-Rouen hearts, clubs, spades and diamonds we all know, there's a separate Page and Knight card, rather than the modern convention of combining them into the Jack, and the trumps are only numbered with Roman numerals rather than named. Each one shows a particular event taking place in the countryside and a similar such event taking place in a town or small city.
The only restriction he'd have with making a tarot deck at all is that fewer publishers will make tarot decks - and USPC is not one of them.