The term has roots in the early centuries of the church, when the Catholic community — living and dead — was envisioned as having three parts. These were later called the Church Triumphant (composed of those in heaven), the Church Suffering or Church Penitent (those in purgatory) and the Church Militant (those on earth).

Catholic teaching held that the spiritual efforts of the Church Militant would hasten the ascent into heaven of the souls in purgatory. But how is a concept that was formed during Roman persecution of early Christians and took on a martial connotation during the Crusades meant to be understood in a democratic, capitalist, polyglot, multimedia society like the modern United States?

“When you heard the expression ‘the Church Militant,’ it didn’t bring to mind a call to arms or some kind of mobilized, militant action in the way we understand the term now,” said John C. Cavadini, a professor of theology at the University of Notre Dame. “A lot of the struggle of the Church Militant is against interior temptations that lead you to greed and all kinds of spiritual pathologies. And it’s about engaging in acts of mercy. Part of the victory of the Church Militant is the victory of love. It didn’t have the triumphalist and militarized connotation that’s been attached to it now.”

While the term remains in the Roman catechism, which was promulgated by the Council of Trent in the mid-1500s, the official catechism produced under Pope John Paul II in 1992 replaced “Church Militant” with “pilgrims on earth.” The adult catechism then devised by Catholic bishops in the United States adopted those words, and they are overwhelmingly the norm in Catholic practice in the United States and abroad.

Patrick J. Buchanan, one of Mr. Trump’s precursors in running for president on a platform of right-wing populism, embraced Church Militant theology in a 2009 essay in the conservative magazine Human Events. After delineating conflicts between Catholic leaders and Democratic politicians over issues like abortion and contraception, Mr. Buchanan made a more sweeping assertion:

“Catholicism is necessarily an adversary faith and culture in an America where a triumphant secularism has captured the heights, from Hollywood to the media, the arts and the academy, and relishes nothing more than insults to and blasphemous mockery of the Church of Rome.”

The words could serve as a mission statement for Mr. Vorsi’s ChurchMilitant.com. A television producer who renounced his earlier life as a gay man, Mr. Voris, 55, has developed a media operation from ChurchMilitant.com’s studio in suburban Detroit that produces books, online articles, YouTube videos, podcasts and a daily talk show. These cumulatively attract about 1.5 million views a month, he said.