Richard Sipe, a former monk from St. John’s Abbey in Minnesota, was one of the nation’s foremost authorities on clergy sex abuse and a thorn in the side of the Catholic Church.

A pioneer in the field of clergy abuse research in the 1990s, Sipe documented the link between priest celibacy — and clergy culture — and the sexual abuse of children. He was an indispensable consultant and expert witness on priest sex abuse cases across the country, including in Minnesota.

Sipe, 85, died at his home in La Jolla, Calif., on Aug. 8 after a long illness.

“Did you see this week that 300 priests were accused of abusing thousands of children in Pennsylvania?” asked his brother Tom Sipe of Robbinsdale. “That’s why he made this his lifelong mission.”

Jeff Anderson, a St. Paul attorney for abuse victims who worked with Sipe for 30 years, said Sipe was among the nation’s most influential voices on abuse “and whose body of [research] work may be the most lasting.”

“He exposed the root causes of the sex abuse scandal, and the scandal itself,” Anderson said.

Walter Richard Sipe, born Dec. 11, 1932, was one of 10 children of Walter and Elizabeth Sipe of Robbinsdale, whose family car repair business remains open today. Sipe attended St. John’s Preparatory School in Collegeville, Minn., and went on to become a Benedictine monk and priest at St. John’s, working as the abbey’s personnel director in the late 1960s.

He also studied at the Seton Psychiatric Institute in Baltimore, earning a certificate in counseling clergy, where he learned about the world of troubled priests. Sipe left the priesthood in 1970 and later married a former Maryknoll nun, Marianne Benkert. He worked as psychotherapist in Baltimore until they moved to California in the 1990s.

In 1990, Sipe published “A Secret World: Sexuality and the Search for Celibacy,” based on case studies and interviews with 1,500 priests. It was groundbreaking work, creating an academic template for documenting clergy abuse.

Sipe went on to pen five more books and nearly 100 articles, as well as to consult nationally with attorneys for abuse victims in some of the nation’s biggest cases.

Through it all, he never lost his faith, his relatives said. Exposing how the church handled child abusers was a moral issue for him, they said.

“He fought an uphill battle his whole life,” said another brother, John Sipe, of St. Paul.

John Sipe said his brother frequently returned to Minnesota, where he both collaborated with Anderson and visited his four younger siblings here. That included an 80th birthday gathering at Forepaugh’s Restaurant in St. Paul.

Family members described him as a modest, good-natured man with a keen intellect and endless dedication to exposing clergy abuse and protecting children.

“We tried to get him to retire,” said John Sipe. “Then someone would throw him another case. His dining room table was stacked 18 inches high with documents.”

When Minnesota’s clergy abuse scandal broke several years ago, Sipe provided insights to journalists, including Star Tribune reporters. He also advised Boston Globe journalists featured in the Academy Award-winning film “Spotlight” and appeared in the Netflix documentary series “The Keepers.”

His death leaves a huge void in the abuse survivor community, advocates said. “I texted him, ‘Who is going to take your place?’ ” said his brother Tom Sipe. “He never responded. I don’t think anyone could.”

Sipe is survived by his wife, Marianne Benkert Sipe, and son, Walter E.B. Sipe, both of California. A memorial service will be held later this month.