It's a powerful and iconic scene in the movie, where the two main characters, Connor and Murphy MacManus, recite a prayer before going on a killing spree. The prayer itself is a combination of actual Latin phrases and some made-up words.
Here's the full text of the prayer:
"Sanctus, Sanctus, Sanctus, Dominus Deus Sabaoth. Pleni sunt caeli et terra gloria tua. Hosanna in excelsis. Deus, Deus meus, ad te de luce. Adorate Deum. Exultate Deo, adjutori nostro. Jubilate Deo Jacob. Domine, Deus, rex fortis, Deus noster, Deus noster, Deus noster. Deus, Deus meus, ad te de luce. Adorate Deum. Exultate Deo, adjutori nostro. Jubilate Deo Jacob. Domine, Deus, rex fortis, Deus noster, Deus noster, Deus noster. Deus, Deus meus, ad te de luce. Adorate Deum. Exultate Deo, adjutori nostro. Jubilate Deo Jacob. Domine, Deus, rex fortis, Deus noster, Deus noster, Deus noster."
While the actual meaning of the prayer is unclear, it's meant to be a powerful and ritualistic call to action for the MacManus brothers, giving them a sense of justification for their actions.