New York ? For nearly half a century, Jocelynn Williams attended Sunday Mass at St. Thomas the Apostle, a predominantly black Roman Catholic church in Harlem. She received her First Communion and mourned at her mother’s funeral there.

Now, Williams and other parishioners pray each Sunday outside the church, its doors padlocked. A “No Trespassing” sign is attached to the iron gate.

The Archdiocese of New York closed St. Thomas in August, a move propelled by a dwindling congregation and steep maintenance costs, a church spokesman said. Embittered parishioners have responded with their Sunday vigils and weekday protests outside St. Patrick’s Cathedral, the seat of the archdiocese.

The closing illustrates the challenges facing the church as it tries to address the needs of black Catholics in America, a group that — while still a vibrant presence in cities — is also spreading into suburban congregations, according to church observers.

“Many black Catholics are worried about the number of parishes and schools that are closing, because they have been such a strong source of support in the black community,” said Sister Francesca Thompson, a Franciscan nun and assistant dean of undergraduates at Fordham University in the Bronx.

The proportion of blacks in the church has remained steady — at least 3 percent of the 66 million U.S. Catholics.

Exact statistics are not kept, especially since “black” is not defined in a single way, said Jamie Phelps, director of the Institute for Black Catholic Studies at New Orleans’ Xavier University, the nation’s only black Catholic college.

However, in the past decade or more, the number of black Catholics has increased by perhaps half a million with the influx of immigrants from the Caribbean, Africa and Latin America, said Phelps, a Dominican nun. And there are still thriving urban black Catholic churches in cities such as New Orleans, Baltimore, Chicago, Los Angeles and Detroit, she said.

But some established parishes have been shuttered as the congregants have moved out of the city.

“Inner-city congregations tend to be losing their Catholic concentration, as black Catholics assimilate in American culture and get a better education and better jobs. And African-American Catholics do what non-Catholics do: They move to the suburbs,” said Mary Gautier, a senior research associate at Georgetown University’s Center for Applied Research in the Apostolate. “It’s a matter of economics and demographics.”

Keeping the doors open at St. Thomas, a neo-Gothic edifice completed in 1907, would require $5 million in repairs, archdiocesan spokesman Joseph Zwilling said. Yet average Sunday attendance has shrunk to fewer than 200, from a high of several thousand in the church’s heyday, he said.

“We understand that there are deep ties to a church building. But we have 413 parishes and we have to be concerned with all of them,” he said.

If St. Thomas’ parishioners are few, they are devoted. Bearing a sign proclaiming, “Religion Before Real Estate,” Williams and a dozen fellow parishioners protested outside St. Patrick’s one recent afternoon, as they do on a regular basis.

“This is my church,” Williams insisted, leaning on her cane on the Fifth Avenue sidewalk. “Why should I go elsewhere?”

Initially serving an Irish community, St. Thomas was designed by the renowned church architect Thomas H. Poole. Its pipe organ and stained glass windows were made by European craftsmen.

The Irish immigrants who built their new house of worship wanted “the most spectacular church,” said Harlem architectural historian Michael Henry Adams. Architecture critic Paul Goldberger calls it “one of the most unusual and special churches in all of New York City.”