PORTLAND, Maine — Bishop Robert Peter Deeley, 67, will become the head of the Roman Catholic Diocese of Portland and spiritual leader of nearly 200,000 people at a Mass Friday afternoon at the Cathedral of the Immaculate Conception. The event will be broadcast live on Catholic television and radio and will be streamed on the diocesan website.

The Massachusetts native to be installed on Valentine’s Day as Maine’s Catholic bishop may be a bit of a mystery to his new flock, but while in Rome his work led to the dismissal of more than 3,000 priests amid the church’s global sex abuse scandal, according to a reporter who covers the Vatican.

A native of Cambridge, Mass., Deeley was born in 1946 to Irish immigrant parents. He served as a parish priest, then in various capacities in the Metropolitan Tribunal, the ecclesiastical court in the archdiocese of Boston, for 20 years. In 2000, he assumed the presidency of the Canon Law Society of America.

Deeley moved to Rome in September 2004 to assist as an official at the Congregation for the Doctrine of the Faith, or CDF, under the cardinal who became Pope Benedict XVI, according to information released by the Maine diocese in December. He returned to Boston in summer 2011 and was appointed an auxiliary bishop in January 2013.

His duties in Rome included investigating clergy sexual abuse around the world, according to a long-time Vatican watcher.

“Deeley spent the decade prior to his 2011 return home as the top deputy on the CDF team which executed Cardinal Ratzinger, then Pope Benedict’s, sweeping purge of priests credibly accused of sex abuse, a push which procured the dismissals of over 3,000 offenders from the clerical state across the global church,” Rocco Palmo wrote on his blog, Whispers from the Loggia, on Dec. 18, the day Deeley’s appointment was announced.

A former U.S. correspondent for the London-based international Catholic weekly The Tablet, Palmo has been a church analyst for The New York Times, Associated Press, Washington Post, Reuters, Los Angeles Times, BBC, NBC, CNN and NPR, according to the biography on his blog.

Deeley did not respond this week to email questions about his job at the Vatican, so it could not be learned by late Thursday if he investigated any former Maine priests while working in Rome.

Bishop Richard J. Malone announced in January 2007 that the names of 15 priests against whom credible allegations of sexual abuse had been made had been sent to Rome for action as required. The CDF is the final arbiter in such cases, according to information posted on the website for the United States Conference of Catholic Bishops.

The last announcement that priests had been removed came in May 2008. At that time, action had been taken on seven of the 15 cases, according to the diocese. Information on how many cases were pending in Rome was not available late Thursday.

The fact that Deeley was chosen to replace Malone, now head of the Roman Catholic Diocese of Buffalo, is not surprising given Deeley’s recent role as advisor to Cardinal Sean O’Malley, head of the Archdiocese of Boston, according to Palmo. Maine, New Hampshire, Vermont and Massachusetts are part of the Boston Metropolitan, headed by O’Malley.

Deeley served as vicar general and moderator of the Curia of the Archdiocese of Boston when he returned from Rome in summer 2011.

As vicar general, Deeley was responsible for the oversight of nearly 2 million Catholics in approximately 290 parishes across 144 communities in the archdiocese, according to information on its website, www.bostoncatholic.org. As moderator of the curia, his primary duties were to care for and to provide coordination of the personnel and efforts of the central ministries of the archdiocese.

The appointment of a bishop begins in an ecclesiastical province or region. Maine is part of Region 1, which includes all of New England.

“Every bishop may submit to the archbishop of his province the names of priests he thinks would make good bishops” according to the USCCB website. “Prior to the regular province meeting, usually held annually, the archbishop distributes to all the bishops of the province the names and curricula vitae of priests which have been submitted to him.

“Following a discussion among the bishops at the province meeting, a vote is taken on which names to recommend,” the website said. “The number of names on this provincial list may vary. The vote tally, together with the minutes of the meeting, is then forwarded by the archbishop to the apostolic nuncio in Washington, who conducts an investigation and sends his materials to the Congregation of Bishops in Rome, which make a recommendation to the Pope.”

According to the USCCB the process can take up to eight months, but the Maine diocese had been without a bishop for more than 18 months when Deeley’s appointment was announced.

Palmo said that in Rome, Deeley was “seen as a formidable character on all sides with a pastoral sense that’s noticeably deepened over recent years.” The blogger described the Maine diocese as one “that’s been especially hard-hit by the Northeastern church’s titanic challenges of these times: a painful drip of abuse revelations followed by traumatic waves of parish closings, and then, last year, the legalization of same-sex marriage.”

While Maine has the lowest percentage of people in the country who claim a religious affiliation, the Catholic Church has the most members of any denomination in Maine. When Malone was installed in 2004, there were 234,000 Roman Catholics in the diocese. Today, there are 193,392, according to information posted on the diocesan website.

The Installation Mass will be televised live by EWTN and the CatholicTV Network starting at 1:30 p.m. It can be viewed online at www.catholictvlive.com and www.portlanddiocese.org. The Mass can also be heard on The Presence Radio Network (106.7 FM in Greater Portland and Lewiston/Auburn, 90.3 FM in Greater Bangor, 89.7 FM in Bath/Brunswick, and 89.5 FM in Augusta). The diocesan website, Facebook page and Twitter feed will also be updated with the latest information, pictures and video from the historic day.