“Our world is broken, but Jesus is not,” Pastor Eddie Bevill of the Parkridge Church told the congregation, in reaction to the statements some students have made about the futility of prayer as a response to gun violence. “We pray that in the midst of the pain we are experiencing, that we can know you, Jesus.”

Pastor Bevill also asked his flock to pray for the suspect in the shooting, Nikolas Cruz, although he did not mention Mr. Cruz’s name or ask that he be forgiven.

At the high school, about a mile away from the church, a group of grief-stricken teenage survivors vowed to change the laws that allowed Mr. Cruz to get hold of an assault weapon that the authorities say he used to slaughter his former classmates.

In a movement that has been building since the massacre last week, student organizers said on Sunday that they would mount a demonstration next month in Washington called March For Our Lives. Their mission is to pivot America’s long-running gun control debate — which tends to flare up with each mass shooting and then dissipate — toward meaningful action.

“We want this to stop. We need this to stop. We are protecting guns more than people,” said Emma González, 18, one of five core organizers, whose impassioned speech at a rally in Fort Lauderdale on Saturday drew national attention. “We are not trying to take people’s guns away; we are trying to make sure we have gun safety.”