Pope Francis today referred to the mass killing of Armenians by the Ottoman Turks in 1915 as a "genocide," prompting the Turkish government to summon the Vatican's ambassador for questioning.

"In the past century our human family has lived through three massive and unprecedented tragedies. The first, which is widely considered 'the first genocide of the twentieth century,' struck your own Armenian people, the first Christian nation," the Pope said April 12.

Francis' reference to the genocide was taken from a common declaration signed by both Pope Saint John Paull II and Supreme Armenian Patriarch Karekin II in 2001.

His comments took place before celebrating Mass on Divine Mercy Sunday, which is a feast instituted by St. John Paul II and celebrated on the Second Sunday of the Church's liturgical Easter season.

Francis offered the Mass for faithful of the Armenian rite in commemoration of the centenary of the "Metz Yeghern," or Armenian "martyrdom." April 24 is recognized in Armenia as the official date of the start of the event.

Many faithful and members of the Armenian rite were present for Sunday's Mass, including Armenian president Serz Azati Sargsyan, Supreme Patriarch and Catholicos of all Armenians Karekin II, Catholicos Aram I and Patriarch Nerses Bedros XIX.

The Pope has kept strong ties with the Armenian community since his time as archbishop of Buenos Aires, and a group of Argentinian Armenians were among those gathered for the Mass.

During the Mass, Francis also proclaimed Armenian-rite Saint Gregory of Narek a Doctor of the Church, making the 10th century priest, monk, mystic, and poet the first Armenian to receive the title.

Widely referred to as a genocide, the mass killings took place in 1915-1916 when the Ottoman Empire systematically exterminated its minority Armenian population who called Turkey their homeland, most of whom were Christians. Roughly 1.5 million Armenians lost their lives.

Turkey has repeatedly denied that the slaughter was a genocide, saying that the number of deaths was much smaller, and came as a result of conflict surrounding World War I. The country holds that many ethnic Turks also lost their lives in the event.

However, most non-Turkish scholars refer to the episode as a genocide. Argentina, Belgium, Canada, France, Italy, Russia and Uruguay are among the 22 nations that formally recognize the massacre as a genocide.

Reports have circulated saying that the Turkish government summoned the Vatican's papal nuncio, Archbishop Antonio Lucibello, for questioning after the Pope's genocide comment.

When CNA phoned the Turkish embassy to the Holy See, they declined to comment, however the apostolic nunciature in Ankara responded by saying that the nuncio had in fact been called.

After Francis made his comments, the Turkish Foreign Ministry released a statement expressing their "great disappointment and sadness" at the Pope's remarks. They said the words signaled a loss of trust and contradicted his message of peace, the Associated Press reports.

The foreign ministry also held that Francis' words were discriminatory, because he only mentioned the pain suffered by Christians, and not Muslims or any other religious group.

In his greeting ahead of Sunday's Mass, Pope Francis noted how "bishops and priests, religious, women and men, the elderly and even defenseless children and the infirm were murdered" in the 1915 massacre, which targeted Catholic and Orthodox Syrians, Assyrians, Chaldeans and Greeks.

Francis also called to mind other tragic events of the 20th century, including the violence perpetrated by Nazism and Stalinism, as well as other mass killings carried out in Cambodia, Rwanda, Burundi and Bosnia.

"It seems that humanity is incapable of putting a halt to the shedding of innocent blood (and) has refused to learn from its mistakes caused by the law of terror," he said, noting that the enthusiasm to end such violence that came at the end of the Second World War seems to be "disappearing."

By the "complicit silence of others who simply stand by," the agenda of those who seek to eliminate others continues, the Pope said.

"Today too we are experiencing a sort of genocide created by general and collective indifference, by the complicit silence of Cain, who cries out: 'What does it matter to me? Am I my brother's keeper?'"

It is both necessary and a duty to honor the centenary of the "immense and senseless slaughter" the Armenians had to endure, Pope Francis said, because when memories fade, evil can enter and make old wounds fester.

"Concealing or denying evil is like allowing a wound to keep bleeding without bandaging it!" he said, and stressed that evil is never something that comes from God.

In a message given to the Armenian community after the celebration, Pope Francis said that to remember the event is the responsibility of the whole world, so that it can serve as a warning not to repeat similar "horrors" in the future.

He expressed his hope that Turkey and Armenia would work toward a greater reconciliation, and prayed that the Mass and proclamation of St. Gregory as a Doctor of the Church would be an occasion for all Christians to unite in prayer.

At the close of the Mass, Catholicos Karekin II spoke in English, saying that the Armenian genocide is "an unforgettable and undeniable fact of history."

The genocide is deeply engrained into the consciousness of the Armenian people, the patriarch said, therefore "any attempt to erase it from history and from our common memory is doomed to fail."

Karekin observed that according to international law, genocide is a crime against humanity that closely intertwines with condemnation, recognition and repatriation for the act, so therefore the Armenian cause is one of "justice."

In the years after the genocide the Armenian Church has never forgotten "the continuous concern, assistance and solidarity of the Church of Rome toward Armenians," he said.

The patriarch then expressed his "deep gratitude" to Pope Francis, praying that he would be strengthened in body and spirit so as to continue his ministry "with renewed dynamism and spiritual courage."