A group of Roman Catholic academics have taken a stand against Donald Trump's presidential bid by publishing an open letter outlining why the business tycoon is "manifestly unfit to be president."

The open letter was published by The National Review and was authored by Princeton University Professor Robert P. George and George Weigel, senior fellow of the Ethics and Public Policy Center.

"Donald Trump is manifestly unfit to be president of the United States. His campaign has already driven our politics down to new levels of vulgarity. His appeals to racial and ethnic fears and prejudice are offensive to any genuinely Catholic sensibility," the statement said.

The academics also criticised Trump's statements to order U.S. military personnel to torture terrorist suspects and to kill terrorists' families, which the Catholic Church is stongly opposed to.

The letter further calls into question Trump's "commitments to the right to life, to religious freedom and the rights of conscience, to rebuilding the marriage culture, or to subsidiarity and the principle of limited constitutional government."

This is not the first time prominent Catholic church leaders have spoken out about Trump's proposed policies, as the Pope himself weighed in on the media mogul's surprising advance to the forefront of the Republican presidential nomination.

"A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian," Pope Francis said. He however clarified that his statement was not intended to influence the Catholic vote in the US but rather as a response to earlier statements made by Trump on key issues.

The outspoken presidential candidate also did not shy away from hitting back at the Pontiff's statement, saying that the Church's leader's comments were distasteful.

"For a religious leader to question a person's faith is disgraceful. I'm proud to be a Christian and as president I will not allow Christianity to be consistently attacked and weakened unlike what is happening now with our current president," he said.

Other signatories of the open letter included was also signed by Ryan T. Anderson of The Heritage Foundation; Gerard V. Bradley, law professor at the University of Notre Dame; Thomas F. Farr, director of the Religious Freedom Project at Georgetown University; and Nina Shea, director of the Center for Religious Freedom at the Hudson Institute.