Pope Francis on Sunday will declare a 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 he utters the word "genocide" during his homily.

Francis is marking the 100th anniversary of the slaughter 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.

It's a big deal for the Armenians, who have been campaigning for greater recognition that the slaughter constituted a 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.