NEW YORK—It’s a group with millions of members and at least 7,000 congregations in more than 100 countries, and it claims to be the only true church. Most Americans don’t know about the Iglesia ni Cristo, or “Church of Christ,” with its gleaming, spire-topped buildings that have popped up everywhere from South Korea to South Africa.

In the last decade this religious sect has expanded rapidly, and it now has about 340 congregations in the United States. It has recently bought entire ghost towns in Connecticut and South Dakota. The Connecticut town will serve, among other things, as an educational center to train ministers.

In New York City last fall, the Iglesia ni Cristo (INC) hosted oratorios at the famed Lincoln Center, and its members have banded together to do disaster recovery in the Philippines, to give to local food banks, and to break Guinness world records. Much of its work looks similar to that of any Christian church, with worship services, meals, voluntary tithing, and community service. It is built on intense communal relationships and has schools, a university, sports activities, and piano competitions.

But the group’s beliefs are non-Trinitarian, and critics also link it with patterns of intimidation and violence toward former or dissenting members—patterns the INC, headquartered in the Philippines, has avoided acknowledging. At least three expelled members of the group have been granted asylum in Canada in the last two years because of reported threats to their lives.

Some others who have been expelled say they have been threatened and stalked for criticizing the sect, which prizes unity and commitment to its central leadership under Eduardo V. Manalo, the grandson of the founder.

“When the members ask questions … you are automatically deemed as a defector, that you’re against the administration,” said Liezl Deocampo, who was expelled from the INC in California in 2015 along with her family.

Deocampo said she has been stalked, with online threats and people parked outside her home, and her sisters cannot visit her for fear of expulsion themselves. She recently bought a gun and took gun training lessons.

The INC has denied every accusation of maltreatment or intimidation of former members.

“These people who are coming up with all these allegations, they were expelled from the church,” said Edwil Zabala, one of the group’s top ministers, who is based in the central office in the Philippines. “It’s just like dealing with a disgruntled ex-employee. We don’t expect them to have good or nice words about us.”