VATICAN CITY — Pope Francis on Friday pleaded with sexual abusers within the Roman Catholic Church to surrender to authority, in his most harsh words yet about the decades-long scandal.

"To those who abuse minors I would say this: Convert and hand yourself over to human justice, and prepare for divine justice," the pontiff said his traditional Christmas address to the Curia, the Vatican's central administration.

In "human justice," it wasn't immediately clear if Francis was referring to the church's own judicial system, secular authority or both.

He said the church cannot stand idly by as predator priests continue to work and their victims live in silent fear.

"It is undeniable that some in the past, out of irresponsibility, disbelief, lack of training, inexperience, or spiritual and human short-sightedness, treated many cases without the seriousness and promptness that was due," Francis said "That must never happen again. This is the choice and the decision of the whole church."

Francis acknowledged the difficulty of catching sexual predators in any setting.

"This is no easy task, since the guilty are capable of covering their tracks, to the point where many wives, mothers and sisters are unable to detect them," according to the pontiff. "The victims, too, are carefully selected by their predators [as] often [they] prefer silence and live in fear of shame and the terror rejection."

Still, the pope promised the church will "spare no efforts" to seek justice for sexual abuse victims.

"Dear brothers and sisters, let it be clear that before these abominations, the church will spare no effort to do all that is necessary to bring to justice whoever has committed such crimes," he said. "The church will never seek to hush up or not take seriously any case."

The Catholic Church has struggled for years to shed accusations that it's covered up for abusive priests within its ranks.

The Illinois attorney general just this week issued a damning report that the identities of more than 500 abusive Catholic priests have been concealed by the church.

"By choosing not to thoroughly investigate allegations, the Catholic Church has failed in its moral obligation to provide survivors, parishioners and the public a complete and accurate accounting of all sexually inappropriate behavior involving priests in Illinois," the state's attorney general, Lisa Madigan, said in a statement.

"The failure to investigate also means that the Catholic Church has never made an effort to determine whether the conduct of the accused priests was ignored or covered up by superiors."