Religion became a point of contention on Monday in the trial of Lindsay Clancy, a Massachusetts mother charged with murder in the strangling deaths of her three young children.

For the second time since the trial opened last month, a reference to “mortal sin,” a concept that is part of the Catholic faith, incensed the judge, who sternly instructed jurors to disregard any references to it.

Ms. Clancy, 36, who was raised Catholic, does not deny strangling her children — Cora, 5, Dawson, 3, and Callan, 8 months — at her home in Duxbury, Mass., an affluent suburb of Boston, in January 2023. But she has pleaded not guilty to three counts of first-degree murder, citing postpartum psychosis. She faces life in prison without parole if convicted.

Her lawyer has argued that she killed her children because she had been suffering from severe postpartum mental illness and had auditory hallucinations of a voice ordering her to do so.

Prosecutors argued that Ms. Clancy had planned the killings rationally and plotted to get her then-husband, Patrick Clancy, out of the house before she choked their children with exercise bands.

Monday’s proceedings were devoted to rebuttals, which give prosecutors an opportunity to respond to claims made by the defense. Those will continue on Tuesday, and closing arguments will follow, possibly on Wednesday. After that, the jury will deliberate to reach a verdict.

The high-profile murder trial began about four weeks ago in Plymouth, Mass. Witness testimony so far has been a jarring mix: fond family memories, creeping signs of psychological distress and horrific crime scene descriptions, along with expert assessments of Ms. Clancy’s mental state at the time of the killings.

The proceedings on Monday were less emotional than they had been for much of the trial, but more contentious.

Kirk Heilbrun, a psychologist and witness for the prosecution who evaluated Ms. Clancy after the killings, said in his testimony that she had reported saying “Go to God” as she strangled her children.

“When I asked her about that, when I said, ‘Isn’t suicide a mortal sin?’” he testified before being cut off by an objection from Ms. Clancy’s lawyer, Kevin Reddington.

The judge, William Sullivan, quickly sent the jury out of the room.

Mr. Reddington then asked him to grant a mistrial because, he said, the prosecution’s witness had made improper references to religion. “This is just over the top,” he said.

Jennifer Sprague, a prosecutor, said she had not asked Dr. Heilbrun for “any testimony about Catholicism or religion.”

Judge Sullivan did not grant a mistrial. But when the jury members returned, his words to them were gruff. Ms. Clancy’s religion “is absolutely an inappropriate area of testimony,” he said. “It is to be stricken. It is not to be considered.”

The exchange echoed a moment last week during the testimony of Ms. Clancy’s former mother-in-law, Susan Clancy. After a series of questions that referenced both women’s involvement with the Catholic Church, Shanan Buckingham, another prosecutor, asked: “So you’re aware that murder is considered a mortal sin?”

The next sound was Judge Sullivan’s gavel. He called for a sidebar and then struck the question from the record.

Referencing religion can be tricky territory in a court of law, said Laurie Levenson, a Loyola Law School professor.

“This is a very dangerous approach by the prosecution,” she said, adding that morality and religious beliefs are “not what governs an American courtroom. What governs an American courtroom is the law.”

Another moment of drama came earlier in the day, during the defense’s cross-examination of the prosecution witness Dr. Avram Mack. Dr. Mack is a forensic psychiatrist who took issue with Mr. Reddington’s phrasing of a series of questions about the symptoms of psychosis.

Mr. Reddington ended his cross-examination abruptly. “I don’t care,” he said, briskly closing his binder. “Thank you.”

Ellen Barry contributed reporting from Plymouth, Mass.