ROME — Rushing to meet his wife at St. Peter’s Square, Thomas Williams walked past priests in black cassocks and said that the only thing he missed from his own decades of wearing a priest’s collar was “a real sense of helping people very directly.”

He paused and added, “I’m sure I’m helping people in some way.”

Mr. Williams’s current mission, since 2014, is in the service of Breitbart News, the populist, right-wing website that backed Donald J. Trump in his run for the presidency and is popular with the alt-right, an extremist and often xenophobic movement that embraces white nationalism.

The website is now hoping to buoy Europe’s surging anti-immigrant parties by spreading into Germany and France. But for years, Breitbart has had a presence in London, Jerusalem and Rome, which is perhaps most important for its imagining of itself as an expanding empire with a foothold in the ancestral home of the Crusades.

To man the fort in Rome, Stephen K. Bannon, then Breitbart’s chief executive and now Mr. Trump’s chief White House strategist, turned to Mr. Williams, a telegenic and polyglot theologian who had spoken for the Vatican and defended the leader of his conservative religious order against accusations of child molestation (ultimately proved true). Mr. Williams himself then left the priesthood in disgrace when it emerged that he had broken his vows of celibacy and fathered a son.