Criteria stating that a jury must be able to sentence a suspect to death means that Catholics may be disqualified from the Boston Marathon bombing trial, it has emerged.

Dzhokhar Tsarnaev's trial has been delayed as the court struggles to find potential jurors without bias, a connection to the case, or religious beliefs that could impact their decision.

According to the catechism of the Catholic Church, the death sentence should not be used when "non-lethal means are sufficient to defend and protect people's safety from the aggressor."

Rev James Bretzke, professor of moral theology at Boston College, told USA Today that it is "frustrating" that this could mean no Catholics will sit in the jury.

"It is both ironic and unfortunate that Catholics who understand and embrace this teaching will be systematically excluded from the trial," he said.

Michele Dillon, a University of New Hampshire sociologist and co-author of 'American Catholics in Transition' also expressed concern. Religious belief "shouldn't be enough to disqualify" candidates she said.

"We're supposed to have a jury of one's peers. And if one's peers are informed by this sort of religious ethos, then that surely deserves some kind of recognition."

Potential candidates for jury duty have been asked to fill out a questionnaire to determine their eligibility. US District Judge George O'Toole is seeking to build a panel of 12 jurors and six alternates to determine whether 21-year-old Tsarnaev was guilty of killing three people and injuring 264 in the attack and, if so, whether he deserves the death penalty.

To be eligible, potential jurors must be willing to consider either execution or life in prison without parole if Tsarnaev is convicted of the largest mass-casualty attack on US soil since 9/11.

The first candidate called, a young man who works in advertising, said he had already concluded Tsarnaev was guilty.

"I live with several other males my age, a very testosterone-driven household, they think it's very cool that I would get to sentence him to death," the man said.

Another candidate, a professor of Catholic theology, said he could vote for the death penalty only if the US prison system had physically collapsed.

"Should the walls come down and we needed to protect innocent lives, then one could enforce the death penalty," said the man, who added such a vote would likely prevent him from getting tenure.

Tsarnaev's lawyers have had two motions denied to move the trial out of Boston, and have filed a third. They argue that the client will not receive a fair trial in the city – 68 per cent of over 1,300 who filled out the jury survey already believe him to be guilty, while 69 per cent are connected to the case in some way.

According to recent data, 46 per cent of Greater Boston's population identifies with Catholicism, though it is not known how many of these are devoted to the faith.

Dillon, however, said that even those who don't practice the faith rigorously are likely to take into account the Church's stance

"If they identify as Catholic, part and parcel of why they do that is because they believe these teachings have a lot of value," she said.

Pope Francis last year called for the abolition of capital punishment and declared life imprisonment "a hidden death penalty".

In a meeting with representatives of the International Association of Penal Law in October, the pontiff said: "All Christians and people of good will are thus called today to struggle not only for abolition of the death penalty, whether it be legal or illegal and in all its forms, but also to improve prison conditions, out of respect for the human dignity of persons deprived of their liberty.

"And this," he continued, "I connect with life imprisonment. Life imprisonment is a hidden death penalty."

(Additional reporting by Reuters)