(CNN) -- Pope Francis spoke out against sexual abuse by Catholic clergy on Monday and said he plans to meet with victims in early June.
In a wide-ranging talk with reporters, he also discussed celibacy in the clergy, saying it's not dogma so "the door is always open."
The Pope fielded questions on these subjects during an informal chat with reporters. He simply walked to the back of the papal airplane, sat down with a group of journalists and took questions for about 45 minutes, answering in Italian.
When the Vatican spokesman suggested he stop after half an hour to rest, the Pope indicated he was happy to continue. Such informality has become a hallmark of Pope Francis.
Stressing that sex abuse constitutes a horrific crime, he told reporters that three bishops are under investigation. One has already been found guilty and a penalty is being considered, he said.
It was not clear whether the bishops are under investigation for alleged abuse, or for purported involvement in some sort of cover-up. None of those clerics were named.
A priest who abuses a child betrays the body of the Lord, the Pope said, according to pool reports. He called for zero tolerance.
"This is very serious," he said. "It is like, by way of comparison, holding a black Mass. You are supposed to lead them to sanctity and instead you lead them to a problem that will last their entire lives."
Among the expected invitees to the meeting are abuse victims from Germany, England and Ireland, and Cardinal Sean O'Malley, the archbishop of Boston.
Terrence Donilon, a spokesman for the cardinal, said the time and date of the meeting have not been finalized.
"Cardinal O'Malley has been asked by the Holy Father to assist with the planning for a meeting with survivors of sexual abuse in the coming months," said Donilon. "The cardinal looks forward to supporting this effort by Pope Francis in whatever manner will be most helpful."
The meeting at the Vatican will not be the first time a pope has met with sexual abuse victims, according to John L. Allen Jr., CNN's senior Vatican analyst. However, it will mark the first time Pope Francis has done so.
"This is a clear indication that Francis is trying to get the message out that he 'gets it' about the need to confront the church's abuse scandals," Allen said.
However, the U.S.-based Survivors Network of those Abused by Priests said the pope's comments did not go far enough.
"No child rape will be prevented, no abuse cover-up will be prevented and no predator priest will be exposed by anything the pope said today or will do next month," Joelle Casteix, western regional director of SNAP, said in a statement. "His upcoming and self-serving meeting with victims is more of what we've seen for decades -- more gestures, promises, symbolism and public relations."
When reporters asked about celibacy in the clergy, Pope Francis showed an open mind but didn't say he's ready to make a change yet.
"The Catholic Church has married priests -- Greek Catholics, Coptic Catholics, those of oriental rites," he said. "Celibacy is not a question of dogma, but rather a rule of life that I greatly appreciate, as I believe it is a gift for the Church. But, since it is not a dogma of faith, the door is always open."
The pontiff spoke as he was returning to Rome from a three-day trip to the Middle East.
During that trip, the Pope extended an invitation to the leaders of Israel and the Palestinian Authority to travel to the Vatican for peace talks.
In comments aboard the plane, he clarified the talks would not focus on finding a solution.
"It will be a meeting of prayer," he said.
The Pope also appeared to open the door to the possibility he might resign one day, like his predecessor, if he no longer had the strength to carry on.
"I think that Pope Benedict XVI was not a unique case," Francis told reporters. "I will do what God tells me to do."
CNN's Eric Marrapodi contributed to this report.