Pope Francis on Sunday will declare a little-known 10th-century Armenian mystic a doctor of the church, one of the highest honors a pope can bestow. More attention, though, is likely to be on whether Francis utters the word "genocide" during his homily.

Francis is marking the 100th anniversary of the killing of an estimated 1.5 million Armenians by the Ottoman Empire by celebrating a Mass in the Armenian Catholic rite in St. Peter's Basilica. The Armenian patriarch, Nerses Bedros XIX Tarmouni, will concelebrate and the Mass will be attended by Armenian President Serzh Sargsyan.

It's a big deal for the Armenians, who in the run-up to the centenary have been campaigning for greater recognition that the slaughter constituted genocide. It's also a big deal for Turkey, which has long denied that the deaths constituted genocide, insisted that the toll has been inflated, and that those killed were victims of civil war and unrest.

Francis avoided the word on Thursday when he met the visiting Armenian church delegation, but said that what transpired 100 years ago involved men "who were capable of systematically planning the annihilation of their brothers."

"Let us invoke divine mercy so that for the love of truth and justice, we can heal every wound and bring about concrete gestures of peace and reconciliation between two nations that are still unable to come to a reasonable consensus on this sad event," he said.

Historians estimate that up to 1.5 million Armenians were killed by Ottoman Turks around the time of World War I, an event widely viewed by genocide scholars as the first genocide of the 20th century. Several European countries recognize the massacres as such, though Italy and the United States, for example, have avoided using the term officially given the importance they place on Turkey as an ally.

According to reports in the Turkish media, Turkey has been working behind the scenes to discourage Francis from uttering the term "genocide" and reportedly successfully campaigned to prevent the papal Mass from being celebrated on April 24, which is considered the actual anniversary of the start of the slaughter.

Last year, Turkish President Recep Tayyip Erdogan issued a message of condolences to descendants of Armenians killed and said Turkey was ready to confront the history of the killings. More recently, Erdogan has accused Armenians of not looking for the truth but seeking to score points against Turkey, saying numerous calls from Turkey for joint research to document precisely what happened had gone unanswered.

The Armenians have found a willing supporter in Francis, who as Cardinal Jorge Mario Bergoglio was particularly close to the Armenian community in Argentina and referred to the "genocide" of Armenians three times in his 2010 book, "On Heaven and Earth."

As pope, Francis provoked Turkish anxiety — and a minor diplomatic incident — when in June 2013 he told a delegation of Armenian Christians that the killing was "the first genocide of the 20th century."

The Vatican spokesman subsequently said the remarks were in no way a formal or public declaration and therefore didn't constitute a public assertion by the pope that genocide took place.

But St. John Paul II referred to the "genocide" both before and during his 2001 trip to Armenia, even signing an official document with the Armenian church leader Catholicos Karekin II noting that that the episode "is generally referred to as the first genocide of the 20th century."

Francis highlighted Turkey's important role as a bridge between faiths during his November visit to the country, and made no reference at all to the Armenian issue — though he did add a last-minute visit to the ailing Armenian patriarch of Constantinople.

On Sunday, Francis will declare the revered mystic St. Gregory of Narek a doctor of the church. Only 35 people have been given the title, which is reserved for those whose writings have greatly served the universal church.

Gregory, who lived around 950 to 1005, is considered one of the most important figures of medieval Armenian religious thought and literature. His "Book of Prayers," also called the "Book of Lamentations," is his best-known work, a mystical poem in 95 sections about "speaking with God from the depths of the heart."