Many people like to buy printed T-shirts, from the silly to the profound. Christians are no exception. And just as a fan of heavy metal might want a shirt on that topic, Christians often like to buy T-shirts espousing their views and beliefs. If you would like to sell Christian-themed T-shirts, there are several approaches beyond just taking out ads.
Obtain images that you can use on your T-shirts as designs. You can either make these yourself, download public domain images or license images from someone else. Then have them printed locally, or open an online store with a company like CafePress or Zazzle to print and sell your shirts.
Contact local Christian stores to see if they would like to either buy your T-shirts outright for resale, or would like to take them on consignment. And you don't have to limit yourself only to stores that sell clothing items. A Christian bookstore might be happy to have a rack of T-shirts that they can offer to customers.
Create flyers and brochures about your products. Make sure they are colorful and well laid out. You want to highlight your best designs or biblical quotations rather than overcrowding the document with everything you have to offer. These flyers or brochures can be sent out as mailers, placed at strategic locations on store counters (in stores that are willing to participate). You might also put them under car windshields in church parking lots. But confirm that this is legal in your area first.