CLEVELAND — The glory days of Holy Ghost Church were years ago, when Catholics packed into the wooden pews, beneath a starry barrel-vaulted ceiling, listening to bells and kissing icons as priests in colorful robes intoned in ancient tongues the liturgies of a faraway land.

The congregation dwindled so much that in 2009 the church was closed, but on a bright Sunday this summer, Holy Ghost was alive again. Mary Matei, visiting from Knoxville, Tenn., snapped pictures on her iPhone as priests sang Mass, while Ann Cogar and Sue Koch, sisters from suburban Cleveland, admired stained glass windows and statuary.

They were taking part in a Mass mob — the latest trend in Rust Belt Catholicism — which is part heritage tour and part mixer (crudités in the fellowship hall followed the service). The movement is bringing thousands of suburban Catholics to visit the struggling, and in some cases closed, urban churches of their parents and grandparents. It is also attracting much-needed donations.

Named after flash mobs — spontaneous gatherings of crowds, often in a public place, to make an artistic or political statement — Mass mobs are spreading around the nation and taking church leaders by surprise. Fueled by social media, they are doing best around Lake Erie: In Detroit, nearly 2,000 people show up to visit churches that normally draw a fraction of that number; hundreds take part in Buffalo, where the movement began; and scores join the events here in Cleveland.