In his wide-ranging press briefing en route from the United States to Rome, Pope Francis spoke on the difficult subject of forgiving priests who have molested minors, saying that the strength to forgive, and to be forgiven, can only come from God.

Francis also told journalists that while he is willing to forgive clergy who have abused children and young people, he understands why there are some who find this difficult.

"We must forgive, because we were all forgiven," the Pope said. However, "it is another thing to receive that forgiveness."

Pope Francis told journalists on board the papal plane he is not judgmental of victims or the families of molested children who struggle to forgive the abuser.

He illustrated this point by recalling a meeting he once had with a victim of molestation, who told him that her mother had "lost her faith and died an atheist" on account of the abuse.

"I understand that woman," the Pope said, "and God who is even better than me understands her."

Pope Francis said he believes this mother "has been received by God," taking into account that it was her own flesh and blood, her daughter, who was molested.

"I don't judge someone who can't forgive," he said, but instead prays for them. "God is a champion in finding paths of solutions. I ask him to fix it."

"What remains is to pray for the Lord to open that door. To forgive, you must be willing."

Pope Francis also stressed that he forgives priests who have abused children, but they in turn must be open to receiving forgiveness.

A priest who has sexually abused a minor, and is not remorseful, "is closed to forgiveness," the pontiff said. "He won't receive it, because he locked the door from the inside."

"If a person has done wrong, is conscious of what he has done, and does not say sorry, I ask God to take him into account."

"I forgive him, but he does not receive that forgiveness. He is closed to forgiveness." Not everyone is able or willing to receive forgiveness, he acknowledged.

"What I'm saying is hard. And that is how you explain how there is people who finish their life hardened, badly, without receiving the tenderness of God."

Pope Francis made these remarks on the papal plane returning from his Sept. 19-28 visit to Cuba and the U.S. While in the United States, he spoke several times on the topic of sex abuse by clergy.

The apostolic journey concluded with his visit to Philadelphia, a city which was struck hard by the clerical sex abuse crisis. While there, the Pope met with five survivors who had been molested as children, either by clergy, family members or educators.

One journalist asked Pope Francis about his remarks to the U.S. bishops in Washington, D.C. about the clerical sex abuse crisis – specifically, his reasons for offering them comfort in the wake of the scandal.

"I felt the need to express compassion because something really terrible happened," he said in reference to the Sept. 23 speech, explaining that many of the bishops who suffered "did not know of this."

The Pope said he made reference to the book of Revelation when he told the bishops: "You are coming from a large tribulation. What happened was a great tribulation."

Although sexual abuse of minors exists in many areas, Pope Francis said it is particularly serious when it occurs at the hands of a priest, whose vocation is to lead children toward God.

"We know the abuses are everywhere in families in the neighborhoods, in the schools, in the gyms," he said.

"But, when a priest abuses it is very serious because the vocation of the priest is to make that boy, that girl grow toward the love of God, toward maturity and toward good," he said.

The abuse of minors is "nearly a sacrilege," he said, and by committing these acts the priest has "betrayed his vocation, the calling of the Lord."

The Pope stressed that this is why the Church is adamant that these crimes of abuse must not be covered up.

During the sex abuse crisis in the U.S., it came to light that a number of US bishops sought to prevent these criminal acts by priests from being exposed.

"Those who covered this up are guilty," the pontiff said, in reference to these bishops.

During his Sept. 22-28 visit to the U.S., Pope Francis also spoke with clergy and religious in New York on the country's sex abuse crisis, in which he acknowledged their suffering in the wake of the scandal.

In his homily at the Sept. 24 vespers in St. Patrick's Cathedral the Pope acknowledged they had come out of the period of "great tribulation," and reminded them that their vocation is to be lived out with joy.