They called the project Nuns and Nones, and they were the “nones” — progressive millennials, none of whom were practicing Catholics. Intended to be a pilot project, the unusual roommate situation with the Sisters of Mercy would last for six months.

The idea was spearheaded by Adam Horowitz, a 32-year-old Jewish man, and the pilot program was guided by Judy Carle, a 79-year-old Catholic Sister of Mercy in the Bay Area. Mr. Horowitz and his friends heard the call after a road trip to visit intentional communities. They were brainstorming ways they could live radical activist lives, lives of total devotion to their causes. They were trying to figure out who was already doing this, and when Mr. Horowitz talked to a minister, it came to him. The answer was nuns.

“These are radical, badass women who have lived lives devoted to social justice,” said Ms. Bradley. “And we can learn from them.”

These are also hard times for the sisters. The average age of a Roman Catholic nun in the United States is close to 80. Convents around the country are closing. The number of nuns in the United States has collapsed from 180,000 in 1965 to below 50,000 today. Sisters are passing leadership at Catholic hospitals and schools to lay people. Some have even begun talking about their mission here in America as being complete.