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(CNN) -- Catholic bishops "will be held accountable" for failing to protect children from sexual abuse, Pope Francis said Monday, his strongest acknowledgment yet of what abuse victims have been saying for decades: that the cover-ups have often been as bad as the crimes.

But without strong action to back up those words, such groups are likely to view Francis' comments as little more than lip service. Vatican officials have so far been reluctant to take action against bishops accused of concealing abuse.

In a homily given during a private Mass with six victims of church sexual abuse, Francis apologized for the abuse and asked for forgiveness.

"I beg your forgiveness, too, for the sins of omission on the part of Church leaders who did not respond adequately to reports of abuse made by family members, as well as by abuse victims themselves," Francis said in the homily, according to a text of the statement provided by the Vatican.

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"This led to even greater suffering on the part of those who were abused and it endangered other minors who were at risk," he said.

In his homily, Francis said that "all bishops must carry out their pastoral ministry with the utmost care in order to help foster the protection of minors, and they will be held accountable."

Despite the strong words, a victims advocacy group, the Survivors Network of those Abused by Priests, or SNAP, said Monday's session failed to advance the cause of preventing molestation by priests, arguing that "no child on earth is safer today because of this meeting."

"Let's not mistake this meeting today for real action," SNAP President Barbara Blaine told CNN. "The meeting today will not make children safer."

"I think that Pope Francis has yet to take strong action that will protect children and he could do that by firing the bishops who have been complicit and who are transferring predators," she said.

Francis has called for "zero tolerance" of sexual abuse by clerics, and last week a Vatican tribunal defrocked Polish Archbishop Jozef Wesolowski after finding him guilty of sexual abuse of minors.

CNN religion commentator Rev. Edward Beck said that was a sign of change.

"So what people have been calling for with regards to the hierarchy needing to accept responsibility and action be taken against them has already begun to happen," he said.

Blaine rejected the notion Wesolowski's defrocking was a major shift, saying church officials had covered up for him in the past and have failed to do enough to facilitate his prosecution in the Dominican Republic, where at least some of the abuse was alleged to have occurred.

Monday's meeting between Francis and the six victims of church sexual abuse was not the first such meeting between a pontiff and survivors, but it was the first of Francis' papacy.

It involved three men and three women from Britain, Ireland and Germany who met privately with Francis for about 30 minutes in his private residence at Casa Santa Marca, Vatican spokesman the Rev. Federico Lombardi told reporters.

The abuse survivors also celebrated Mass with Francis.

Lombardi said the victims emerged from the meeting moved by what he described as a "profound spiritual encounter and dialogue."

"If you had seen the people coming out of the meeting, it would have been totally, absolutely clear that it was not a publicity stunt," Lombardi said, reacting to a statement last month by SNAP calling the meeting "self-serving."

"The most important thing the Pope hopes to come out of this occasion, is that the victims themselves felt welcomed and listened to," Lombardi said.

In his homily, Francis acknowledged that sexual abuse by priests has had profound consequences on victims.

"Many of those who have suffered in this way have also sought relief in the path of addiction," he said. "Others have experienced difficulties in significant relationships, with parents, spouses and children. Suffering in families has been especially grave, since the damage provoked by abuse affects these vital family relationships."

"Some have even had to deal with the terrible tragedy of the death of a loved one by suicide," Francis told the victims. "The deaths of these so beloved children of God weigh upon the heart and my conscience and that of the whole Church."

Francis became Pope in March 2013. While critics have said he should have met with victims earlier, Francis has said he believes the Church has been unfairly attacked.

The Pope asked Cardinal Sean O'Malley, the archbishop of Boston, to help organize the meeting.

O'Malley is a member of a new commission spearheaded by Francis -- the Commission for the Protection of Minors -- to advise him on ways to help move forward after the Church's sex abuse crisis.

One member of the group is a woman who was a sex abuse victim.

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CNN's Hada Messia, Delia Gallagher and Eric Marrapodi contributed to this report.