Tina Hoffman, a real estate broker, described Mr. Keller as a local character who enjoyed stirring up controversy. “I thought it was kind of harsh,” she said. “But that’s just Crockett. You got to know Crockett. I don’t think that’s the flavor of our county. This is a very prayerful community.”

The Texas Department of Public Safety, which certifies instructors to teach and train those seeking to qualify for a concealed handgun license, is investigating Mr. Keller’s statements. “Certified instructors are required to comply with all applicable state and federal statutes,” the agency said in a statement. “Conduct by an instructor that denied service to individuals on the basis of race, ethnicity or religion would place that instructor’s certification by the department at risk of suspension or revocation.”

Mustafaa Carroll, the executive director of the Houston chapter of the Council on American-Islamic Relations, found Mr. Keller’s remarks both sad and comical. “It’s more of the same type of anti-Muslim rhetoric that we hear on a fairly regular basis,” said Mr. Carroll, a Muslim who has lived in Texas for three decades. “Outside of looking at television, I doubt if he’s ever seen a Muslim. Muslims are Americans. They pay their taxes. They raise their children. They’re the kind of people you want living next door to you.”

Mr. Keller said that the majority of phone calls he has received have been supportive, and that he does not believe he has done anything wrong. Though the thrust of his radio ad might suggest otherwise, he is a polite and courteous man. Born in Austin, he lives on his great-great-grandfather’s ranch on a remote stretch of U.S. Highway 87, behind Keller’s Riverside Store. He is a member of the Texas Concealed Handgun Association, and he said that throughout his six years of teaching the course, he has never turned a potential student away.

“When you’re in Texas, No. 1, you don’t ask a guy how big his ranch is or how many cattle he runs,” Mr. Keller said. “You really don’t even ask him the name of his wife or his dog. Those are things that are really none of your business. You don’t ask him his religion nor do you ask him his politics. I don’t care what your religion, what your creed is. That makes no bearing. But when people consider themselves a particular religion that has proven itself to be anti-American, well, then, I’m anti-them.”

The class that the ad promoted was held at his store last Wednesday. It was not exactly packed. “We had six people,” he said.