Saying nothing is beyond the reach of God's mercy, Pope Francis told Catholics worldwide he is allowing all priests to absolve the faithful of abortion — women and health workers alike — even while stressing that it is a grave sin in the eyes of the church to "end an innocent life."

In an Apostolic Letter made public Monday, Francis said he was extending indefinitely the special permission he had granted to all rank-and-file priests during the just ended Holy Year of Mercy.

"There is no sin that God's mercy cannot reach and wipe away when it finds a repentant heart seeking to be reconciled" with God, the pope wrote in the 10-page letter, signed Sunday, the day the Holy Year ended.

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But, he added: "I wish to restate as firmly as I can that abortion is a grave sin, since it puts an end to an innocent life."

Because the Roman Catholic Church holds abortion to be such a serious sin, absolution had long been a matter for a bishop, who could either hear the woman's confession himself or delegate it to a priest considered an expert in such situations, a potentially intimidating scenario for many of the faithful.

In his letter, the pope appeared to acknowledge that. "Lest any obstacle arise between the request for reconciliation and God's forgiveness," he wrote, "I henceforth grant to all priests, in virtue of their ministry, the faculty to absolve those who have committed the sin of procured abortion."

A top Vatican official, Monsignor Rino Fisichella, told a news conference at the Vatican on Monday that the pope's words applied to all those who were involved in an abortion — "from the women to the nurse to the doctor and whoever supports this procedure."

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"The sin of abortion is inclusive. Thus forgiveness for the sin of abortion is all-inclusive and extends to all those who are participants in this sin," Fisichella said.

The pope is "absolutely not" lessening the gravity of the sin of abortion, Fisichella added in comments to Sky TG24.

Still, the head of an Italian anti-abortion group expressed concern that some priests might trivialize the sin of abortion. Gian Luigi Gigli, president of the Movement for Life, said women or health care workers who confess to abortion should be given penance in the form of volunteer work at the group's centres that work to prevent abortion.

By permitting all priests to absolve the sin of abortion, Francis was further applying his vision of a merciful church called to minister to the problems of its flock, reflecting concerns he became familiar with while archbishop of Buenos Aires in his native Argentina. Last year, he wrote that some women who had abortions felt they had no choice but to make "this agonizing and painful decision."

In his Apostolic Letter, Francis called on every priest to "be a guide, support and comfort to penitents on this journey of special reconciliation."

O. Carter Snead, director of the Center for Ethics and Culture at the University of Notre Dame, noted that priests hearing the confessions of those involved in abortion had already been "a longstanding practice in the United States and several other countries."

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Thus, Francis is essentially "reminding us that the core message of the right-to-life movement is one of radical hospitality, mercy and unconditional love for every member of the human family, including mothers and fathers whose lives have been broken by abortion and who now seek forgiveness," Snead said.

During the recent U.S. presidential election campaign, some pastors urged their congregations to keep in mind the "sacredness in life" — seen as a reference to abortion — when deciding which candidate would get their vote. U.S. President-elect Donald Trump voiced opposition to abortion during his campaign, while his Democratic opponent, Hillary Clinton, supported women's right to have an abortion.

In his letter on abortion, Francis made plain that there can be no ambiguity in laying out moral principles, even while stressing the church's merciful side.

Addressing priests, Francis wrote: "I ask you to be welcoming to all, witnesses of fatherly love whatever the gravity of the sin involved, attentive in helping penitents to reflect on the evil they have done, clear in presenting moral principles, willing to walk patiently beside the faithful on their penitential journey, farsighted in discerning individual cases and generous in dispensing God's forgiveness."

"Mercy cannot become a mere parenthesis in the life of the church; it constitutes her very existence," Francis wrote.

Four cardinals, including archconservative U.S. Cardinal Raymond Burke, recently criticized Francis for his statements on another hotbed issue in the church, whether divorced Catholics who remarry can receive Communion. Burke and the others expressed fear that Francis was causing "confusion" by saying the matter could be left to the discernment of local priests. Church teaching holds such Catholics are adulterers living in sin, and thus shouldn't receive Communion.

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While the pope's approach to forgiveness for abortion could distress Catholics holding a more rigid application of church teaching, Francis also reached out to traditionalists in his letter.

Faithful attending churches officiated by priests of an ultra-conservative breakaway group, the Fraternity of St. Pius X, can "validly and licitly receive the sacramental absolution of their sins," the pope wrote. Francis had previously granted that permission only for the period of the Holy Year.

Francis also expressed hope that these breakaway priests would strive to work for full Communion with the Roman Catholic Church.