VATICAN CITY - Pope Benedict XVI strongly defended celibacy for priests as a sign of faith in an increasingly secular world Thursday, insisting on a church tradition that has increasingly come under scrutiny amid the clerical sex abuse scandal.

Benedict didn't directly mention the crisis that has rocked the Catholic Church for months in his comments to some 15,000 priests who massed in St. Peter's Square. But in an apparent reference to the crisis, Benedict spoke about "secondary scandals" that showed "our own insufficiencies and sins."

Benedict's comments came during an evening vigil service to mark the end of the Vatican's 'Year of the Priest' - a year that has been marred by revelations of hundreds of new cases of clerical abuse, cover-up and Vatican inaction to root out pedophile priests.

There had been speculation that Benedict might again refer to the scandal during the priestly gathering, following his recent comments en route to Portugal during which he acknowledged that it was born of the "sin within the church" and not from outside elements. Previously, Vatican officials, Vatican publications and cardinals had blamed the scandal on the media, the Masons and anti-Catholic lobbies, among others.

But Benedict didn't directly address it Thursday night. He was due to celebrate a final Mass on Friday before the three-day priest rally comes to a close.

The main U.S. clerical abuse victims group, Survivors Network for those Abused by Priests, said Benedict had passed up a perfect opportunity to announce clear steps to protect children and insist that abusive priests be reported to police.

Benedict responded to preselected questions from five priests and none asked for his thoughts about the scandal. One asked him to speak instead about what he called the "beauty of celibacy," which he said was so often criticized in the secular world.

The pope acknowledged that celibacy was itself "a great scandal" in a world where people have no need for God. But he called it "a great sign of faith, of the presence of God in the world."

Against the so-called scandal of such faith "there are also secondary scandals, that of our own insufficiencies and sins that hide the true scandal," he said.

The Vatican has long denied that its celibacy requirement was the root cause of priests who rape and sodomize children. Yet the requirement - which is a tradition in the church but is not part of church doctrine - has come under increasing questioning amid the scandal and even some top prelates have called for an honest examination of it.

While the pope didn't directly address the crisis, priests visiting Rome for the rally spoke openly about it, saying it was painful - even shameful to them since it reflected badly on all of them.

"Well, I think it was really first a matter of pain, of sadness then a bit of shame because in Belgium we had bishops, not priests who had to resign," said Belgian priest the Rev. Jean Pierre Herman.

"The church isn't perfect. Priests are men. Among priests there are those who will become saints, there are good priests and there are criminals as well. So it happens," he said.

Said the Rev. Fernando Cerero from the diocese di Coatzacoalcos in Mexico, "We felt much shame and sadness, but this is an opportunity (for priests) to reflect on our ministry."