KRAKOW, Poland — The Rev. Tadeusz Isakowicz-Zaleski was tortured by Poland's communist-era secret police. Now he is leading the drive to expose clergy who cooperated with the secret services, saying the church must repent for the misdeeds of compromised priests.

Poland's powerful Roman Catholic Church is reeling from the abrupt resignation of Warsaw Archbishop Stanislaw Wielgus, who admitted cooperating with the hated secret police in the 1970s. Wielgus dramatically stepped down Sunday at what was to have been his opulent installation Mass, stunning deeply Catholic Poland.

Now, church leaders are bracing for the impending release of Isakowicz-Zaleski's book about the secret police's penetration of the church in Krakow. The reverend pored through secret police archives kept by Poland's Institute of National Remembrance and discovered that 39 Krakow priests had collaborated with the regime. Four of them are now bishops, he said.

"The whole tragedy is that the church had 16 years to take care of the problem, and it didn't do a thing," said Isakowicz-Zaleski, 50, dressed in black and sporting a thick beard.

"For many faithful, the problem is not that a priest collaborated. If he were to admit it and ask for forgiveness, the issue would be closed," he said. "The problem is that there is a conviction that the church is hiding a difficult problem."

The widening scandal threatens to tarnish the Polish church, which prided itself on its resistance to the Communist leadership personified by the late Pope John Paul II — the former archbishop of Krakow. John Paul encouraged Poles to challenge the regime peacefully, and many credit him for hastening its 1989 demise.

Yet part of the church's reluctance to acknowledge the collaboration of some priests loops back to the Polish-born pope, Isakowicz-Zaleski said.

"Some said that as long as the pope is alive, you can't smear him. They said the Holy Father did so much for Poland, and so you shouldn't reveal agents so as not to cause any unpleasantness," Isakowicz-Zaleski said.

He noted that the first allegations that a Polish priest collaborated with the regime surfaced three weeks after John Paul's death in April 2005.

Historians say up to 15 percent of Poland's 25,000 clergy may have cooperated with security agencies, informing on church decisions and the attitude of key Catholic leaders at a time when priests were the most persecuted group in the country.

Secret police agents were blamed for the 1984 murder of the Rev. Jerzy Popieluszko, a charismatic Warsaw priest tied to the pro-democracy Solidarity movement.

That was a year before Isakowicz-Zaleski was twice beaten — first in April, then in December. His assailants have never been identified, "but it was a known fact they were secret police agents," he said.

Isakowicz-Zaleski said the attacks were likely a warning to the main Solidarity-aligned priest at Nowa Huta, a community near Krakow where Isakowicz-Zaleski was helping out.

Two years ago, on a train to the Baltic port city of Gdansk for a gathering of former activists of the Solidarity trade union, a friend told Isakowicz-Zaleski that there were secret police files on him from the 1980s at the Institute of National Remembrance.

He went to have a look.

"I was shocked," he said. "There were 500 pages of documents. Everything — passport applications, informant reports on me, secret police reports."

For years, church leaders had believed the assurances from Gen. Czeslaw Kiszczak, an interior minister in the communist regime, that the secret police had destroyed all the files on the church. It turned out that microfilm copies survived — eventually helping bring down Wielgus.

Isakowicz-Zaleski said he told his superiors about his own file, warning that it was an indication that documents about informant priests also had survived.

"Nobody wanted to listen," he said.

Instead, church leaders pressured him to keep quiet. For a while last year, Cardinal Stanislaw Dziwisz ordered Isakowicz-Zaleski not to speak with reporters. Cardinal Jozef Glemp publicly criticized him, accusing him of "sniffing around and tracking down priests to add to his book."

"The church leaders have treated it like it was written by the devil himself," Isakowicz-Zaleski said.