Even i'm not religious - i always enjoy when i'm getting stamps with the pope on it! MY grandmother was a catholic and comes from Austria/Slovenia (boarder region - so we don't know exactly if she is slovenian or austrian because of the 2nd ww) well she always gets envelopes from Austria with the pope on it as i was a child! So i think its because of this.
This day i received this holy gold shimmering stamp from Polska.
Nice enough to show it here. Maybe it's because he's a holy one? ;)