I bet deep down he's an evangelist who thinks he can turn you on to God. The more athiestic you appear, the harder he will try and convert you. BUT if you send confusing religious symbols, you might get him to back off. These guys often think that the Catholic church is next door to Satan himself.
How about buying a few desk ornaments to confuse the heck out of him? Go in to one of those religious retail stores and get yourself a selection. Like an iconic picture of Mary with the bloody heart...and a buddha.