Khaled Yacoub, a parishioner at St. John’s, said he had not gone to the church for a long time out of fear, but decided to attend Christmas services with his wife and two children after hearing assurances that he would be safe. “During the Mass, we heard explosions nearby,” he said.

The priest said he would shorten the liturgy so the worshipers could leave early. While taking pictures in the church garden with his children, Mr. Yacoub said, he heard a huge explosion in the parking lot as people were walking to their cars.

“People were running around,” Mr. Yacoub said. “I caught my kids and entered the church. They were crying.” A woman in the church who had been wounded in her legs was asking for help, he said.

He said, weeping: “The priest was talking about peace. He told us that we have to be Iraqi before Christians, and we must love each other.”

His wife, Sahar Yousif, said: “I wasn’t encouraging the Christians to leave the country, but today am rethinking. I do not know who was behind this targeting, but we will not believe the words of brotherhood and peace and coexistence in Iraq anymore.”