On any other Sunday, Frank Pomeroy, the pastor at First Baptist Church of Sutherland Springs, Tex., would have been in the pulpit. He would have seen the gunman, his steely gaze familiar, barge in mid-sermon. He would have heard the gunfire break out.

But he was hundreds of miles away. And so Mr. Pomeroy, reflecting in his first extensive interview on the mass shooting that took place inside his church, can only imagine the awfulness of it. And ponder whether he could have made a difference had he been preaching that day.

Instead, Mr. Pomeroy was attending a class in Oklahoma City on the morning of Nov. 5. A three-word text message came across his cellphone. “Shooting at church,” it said.

He thought the sender, who was the church’s videographer, was kidding. “I hope you are joking,” he wrote back.