Is television a good marketing tool for a Christian T-shirt business?
Yes . . . that's a tall order. What makes this even more complicated is that the best direct-response agencies and producers have very high minimums. So you'll be largely on your own to oversee the creative development.
Consider hiring a media-buying company to purchase the time. It will be compensated by commission on the schedule you place. As for getting large sales immediately, that depends on getting all of the above right.
Best wishes for success!