Just before landing in Portugal for an official four-day visit, Pope Benedict told journalists aboard his plane that the crisis of child sexual abuse by priests has made the Church realize what he called the "terrifying truth" that its greatest threat comes from "sin within the Church."

In perhaps the pontiff's most revealing public comments since the abuse scandal broke two months ago, Benedict said the Church had "a very deep need" to acknowledge that it must do penitence for its sins and "accept purification."

The pope said "the greatest persecution of the Church does not come from enemies on the outside, but is born from the sin within the Church."

In recent weeks, a number of Vatican officials have accused the international media of waging a smear campaign against the Catholic Church.

Facing the worst crisis of his five-year papacy, Benedict said the Church had to seek forgiveness from the victims of sexual abuse, but also noted that "forgiveness cannot be a substitute for justice."

Over the weekend, the pope accepted the resignation of German bishop Walter Mixa, who has admitted physically abusing children when he worked at a children’s home several decades ago. Last Friday, authorities said they were investigating allegations of sexual abuse against Mixa. A Vatican spokesman said the pope had been informed of those allegations before Mixa submitted his resignation.

Portugal: Catholic but secular

Although the sex abuse scandal has cast a dark shadow over the Church, the highlight of the pope's trip to Portugal is a visit to the Shrine of Fatima, where the Virgin Mary is said to have appeared to three children six times in 1917.

Around 83 percent of the country's residents consider themselves Catholic, a number second only to Poland, but fewer and fewer people attend church. The secular government has recently liberalized its laws on abortion and divorce, and parliament has moved to approve legislation that would allow gays and lesbians to marry.

The pontiff’s visit attracted only small crowds initially, with fewer than 400 people gathering to watch him enter the Presidential Palace after a welcome ceremony at the Jeronimos monastery.

"The lack of people on the streets may be linked to the latest news about pedophilia - as well as a lack of faith among the Portuguese," Angelica Guiao, who was among those gathered to see the pope outside the palace, told the news agency Deutsche Presse Agentur.

But several thousand attended his open-air mass on Tuesday evening in central Lisbon. During his homily, Pope Benedict told the crowd to proclaim their Christian faith "with vigor and joy."

“Today as you play your part in building up the European community, you offer the contribution of your cultural and religious identity,” Benedict said.

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Editor: Chuck Penfold