LISBON (Reuters) - Pope Benedict said on Tuesday that the greatest threat to Catholicism came from “sin within the Church”, one if his most forthright comments so far on a sexual abuse scandal that has created turmoil in the church.

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The Church has “a very deep need” to recognize that it must do penitence for its sins and “accept purification”, he said.

“Today we see in a truly terrifying way that the greatest persecution of the Church does not come from outside enemies but is born of sin within the Church,” Benedict told reporters on the plane to Portugal, replying to a question about the scandal.

In recent weeks, a number of Vatican officials have accused the media, gays or progressives of waging a smear campaign against the Church. One top Vatican official even dismissed reports of a cover-up of sexual abuse of children by priests as “petty gossip”.

The 83-year-old German pontiff, facing the worst crisis of his five-year-old papacy, said the Church had to seek forgiveness from victims of sexual abuse but also recognized that “forgiveness cannot be a substitute for justice”.

The pope promised abuse victims he met in Malta last month the Church would do all it could to investigate allegations, bring to justice those responsible for abuse and implement effective measures to protect young people in the future.

But the scandal shows no sign of abating. On Saturday Bishop Walter Mixa of Augsburg, who has been accused of sexually abusing minors, became the first bishop in the pope’s native Germany to step down over the scandal.

In recent weeks, a Belgian bishop resigned after admitting he had sexually abused a boy and three Irish bishops quit over their handling of sexual abuse cases.

ECONOMIC CRISIS

In his comments on the plane, the pope also mentioned Portugal’s economic crisis. The minority socialist government is struggling to reduce a huge deficit but harsh austerity measures will impose greater sacrifices to avoid a Greek-style debt crisis.

Benedict, who was due to hold a large outdoor Mass on Tuesday afternoon, repeated his call for greater moral responsibility in financial decisions and acknowledged the Church should in the past have spoken out more on economics.

“We must admit that the Catholic faith ... was often too individualistic. It too often left concrete things to the world and thought only of individual salvation and religious affairs without realizing that there was a global responsibility (for economic decisions),” he said.

The main purpose of the pope’s four-day visit to Portugal is to visit the shrine at Fatima where the Madonna is said to have appeared to three shepherd children six times in 1917.

One of the three messages the Madonna is said to have given to the child visionaries -- the so-called ‘Third Secret of Fatima’ -- was what the Vatican has said was a prediction of the 1981 assassination attempt against the late Pope John Paul.

Benedict told reporters he believed that interpretation of the Third Secret, revealed in 2000, could include the suffering the papacy and the Church would have to endure as a result of today’s sexual abuse crisis. (Editing by Tim Pearce)