The Vatican is set to launch a new structure today that will allow Anglican parishes in the United States, and their married priests, to join the Catholic Church in a small but symbolically potent effort to reunite Protestants and Catholics, who split 500 years ago.

More than 1,300 Anglicans, including 100 Anglican priests, have applied to be part of the new body, essentially a diocese. Among them are members of St. Luke’s in Bladensburg, Md., which this summer became the first group in the country to convert to Catholicism.

St. Luke’s was part of the Episcopal Church, the official wing of American Anglicanism. But most of those joining the new structure are Anglicans who aren’t part of the Episcopal Church.

It’s unclear how many priests and their followers will ultimately convert to Catholicism. Compared with the tens of millions of Americans who identify as Catholic or Protestant, the movement is small. But it is the most tangible progress in decades for Catholic leaders, who see Catholics and Protestants as estranged siblings who should be reconciled.

“It’s the largest reunification effort in 500 years,” said Susan Gibbs, a spokeswoman for the new body, called an ordinariate.

The possibility of dozens of married Catholic priests could provide fodder for Catholics who want the Vatican to open up on the issue of priestly celibacy. There are about 40,000 Catholic priests in the United States.

Gibbs declined to say which priests and parishes have expressed interest. But congregants at St. Luke’s, and others who call themselves Anglo-Catholics, tend to be theological and social conservatives who say they like the clear, single authority of a pope. However, they also want to hold on to aspects of Anglicanism, including retaining more authority in governing and certain music and rituals.

More details will be made public today but, Gibbs said, most of the Anglicans who expressed interest were not those leaving the Episcopal Church. Most belong to offshoot Anglican groups, many of which have grown since the Episcopal Church ordained an openly gay bishop in 2003.

Tens of thousands have left the Episcopal Church since then for breakaway groups.

But people in both movements, Anglo-Catholics and Episcopal breakaway groups, tend to voice similar concerns about the liberal direction of the Episcopal Church. They mention the ordination and marrying of gays and lesbians, the ordination of women, and leaders who view the Bible as metaphor, not fact.

In 1980, the Vatican created a different system for American Anglicans to join the Catholic Church but didn’t give them as much freedom as the new structure provides. Since then, 90 Anglican priests have joined the Catholic Church, as have seven congregations, totalling 1,230 families, Gibbs said. Almost all are in Texas and Pennsylvania.

The ordinariate is the second. One launched earlier in 2011 in England has some 1,000 members and 57 priests, Gibbs said.

Gibbs said that the movement wouldn’t change the church’s position on celibacy and that the exception is only for married Anglican priests.

The Very Rev. Thomas Ferguson, dean of Bexley Hall Seminary in Columbus, Ohio, said the change expands and formalizes the process by which Episcopalians and Anglicans have been able to join the Catholic Church.

He said he expects most who convert under the new changes, perhaps 1,000 to 2,000 people, will be from groups that broke away from the Episcopal Church years ago. He said the Episcopal Church has about 2 million members.

“In the end, we’re a country founded on religious beliefs, and people need to go where they’re called to go,” Ferguson said. “That’s fine. God bless them.”

But Ferguson, who worked from 2001 until 2011 as one of the Episcopal Church’s chief liaisons with other Christian churches and world religions, said he found it “disconcerting” that the Catholic Church “just kind of announced” the changes before consulting Episcopal leaders.

“If this papacy sees this as the only way to dialogue with other religions,” Ferguson said, “that’s troubling.”