WALDOBORO, Maine — With millions of people having stayed home from places of worship during the coronavirus pandemic, struggling congregations have one key question: How many of them will return? As the pandemic recedes in the United States and in-person services resume, worries of a deepening slide in attendance are universal. Some houses of worship won’t make it. Smaller organizations with older congregations that struggled to adapt during the pandemic are in the greatest danger of a downward spiral from which they can’t recover, said the Rev. Gloria E. White-Hammond, lecturer at the Harvard Divinity School and co-pastor of a church in Boston. On the Maine coast, the pandemic proved to be the last straw for the 164-year-old Waldoboro United Methodist Church. Even before COVID-19 swept the world, weekly attendance had dipped to 25 or 30 at the white-clapboard New England church that could hold several hundred worshippers. The number further dwindled to five or six before the final service was held Sunday, said the Rev. Gregory Foster.Millions skipped church during pandemic. Will they return?