It was a Saturday, not a Sunday, when the former St. William's church had people back in its pews and songs that could be heard out on Dorchester Avenue.

Over 300 people, worshippers from Dorchester, Roxbury, Rhode Island, New York and Cape Verde, came for the first service in the ex-Catholic church, now in the care of Seventh-Day Adventists from Roxbury.

"We are here because God is good," said Pastor Samuel Bulgin, welcoming his congregation to the first service at the church. A chorus of "Amen" followed.

The church as a home for the Seventh-Day Adventist congregation will have its official opening on the second weekend in September, almost exactly four years after Catholic parishioners held their last Mass, carrying out a statue of St. William in a march around the block.

Rebuilt after a fire destroyed an original church structure in 1980, St. William's was the only one out of 11 parishes in Dorchester to be closed outright in an Archdiocesean reconfiguration. It was sold to the Vietnamese American Initiative for Development, a Fields Corner-based community development corporation, for $2 million. The Seventh-Day Adventist church bought St. William's from Viet-AID earlier this year for $2.5 million and will have to pay $150,000 annual mortgage.

It remains unclear what will happen to the old parish school building on Savin Hill Avenue, which held its last classes in June. Church officials did not return phone calls seeking comment.

"I know that God will come through," Bulgin said, before children set out to crisscross the pews, armed with woven baskets and plastic cups.

The Seventh-Day Adventist Church, a Protestant Christian denomination, celebrates the Sabbath on Saturdays. At the day-long service last Saturday, worshippers fanned their faces as Bulgin and others spoke and sang, and acted out stories of prodigal sons for the children.

The air conditioning was not working; the pulpit, along with chairs on the stage, was borrowed from another church. Church members had come in to scrub the green carpet clean days before, working Thursday afternoon and into the night. Part of the sanctuary has been repainted, along with the fellowship area.

"We probably just need to paint the hallways and complete servicing on the AC unit," Bulgin said later in a phone interview. He said the transfer was going "more smoothly than I thought it would."

His congregation - which presently numbers about 200 - have been renting a church in Roxbury for eight years. References to the Roxbury church peppered the service and its materials, including the service's beginning, with a unicorn and a "Welcome to Roxbury" banner projected onto the front wall.

"It's nice. It's in the community. That's needed," said worshipper Damian Brown, who lives 10 blocks away. He has been going to an Adventist church in Mattapan. "It's spacious," he added. "It just needs the AC to work."

Not everybody was pleased with the service.

Standing outside as the new congregation sang hymns, Mary DiBiase stopped to speak with her Victoria St. neighbors and to survey the multitude of cars that lined the streets.

"They're going to need to do something," she said. "They're very nice people, but they have to understand there's no parking and we were here before them. They don't pay taxes like we do. They're going to have to put them on the roof or somewhere."

A former St. William's parishioner who now goes to Blessed Mother Teresa's on Columbia Road, DiBiase also expressed concerns about the potential of more members joining up with the church.

"You can't put a steak in a can of sardines," she said. "And that's what they're trying to do."

Pastor Bulgin, who met with several abutters on Monday, said the church was working with the community to develop some space for parking.

"I think it's a shared concern," he said.

Some church officials said they are exploring the possibility of using the small park behind the church as a parking lot and also renting a spot nearby and busing worshippers in.

"It's something we're going to work out," said church Elder Richard Watkins.

Dorchester isn't alone among communities having to deal with the results of a contracting Catholic population and new congregations moving in.

"This is part of a national trend," said Alan Wolfe, founding director of the Boisi Center for Religion and American Public Life at Boston College.

A publication, "Adventist World," available at the front of the church touted a million new believers who had joined the main church in the past 12 months.

Halfway through Saturday's service, a silver car pulled up, and a small 85-year-old woman emerged, carefully making her way inside the church as tears streamed down her face.

An Adventist minister met her and she hugged him, resting her small head in the crook of his arm.

Between tears, Catherine White of Savin Hill said she was married in St. William's church and her son was ordained there.

"I'm glad it's still a church," Catherine Coyne said, helping her mother back into the car after she received a tour from the minister and echoing a sentiment felt by some of the abutters. "It's better than a Chinese restaurant."