According to Geanakoplos (Byzantine East and Latin West) a large community of Greek monks seems to have always been living in Rome. Besides that, until the 3th century Greek was the language of the Roman liturgy. Even today, in the office of the Great Friday in the Roman church one can listen a chant in both languages, first in Greek (Aghios Athanatos Eleison Emas) and then the same in Latin (Sanctus immortalis miserere nobis). Geanakoplos mentions also Wellesz (Eastern Elements in Western Chant), who writes about the use of the 3 Aghios (=sanctus) in the Roman church, until today.
Constantelos (Greek Orthodoxy – From Apostolic Times to the Present Day) writes that for several centuries the worship in the Latin speaking West was in Greek. Writing about the Roman Liturgy, C.E. Hammond, a renown liturgiologist of the last century adds: “it is, we believe, acknowledged on all sides [history, archeology, literature and criticism] that the language of the early Roman Church, i.e. of the first three centuries, was Greek.” In full agreement he cites his contemporary ecclesiastical historian Henry Hart Milman who writes: “For some considerable (it cannot but be an undefinable) part of three first centuries, the Church of Rome, and most, if not all the Churches of the West, were, if we may so speak, Greek religious colonies. Their language was Greek, their organization Greek, their writers Greek, their scriptures Greek; and many vestiges and traditions show that their ritual, their Liturgy, was Greek.”
Aeneas’ Quest blog picked 3 characteristic selections of early chant in Rome. Listen to all of them and, if you ever happened to be in a Greek church for the Liturgy, or if you just have listened to a recording of Greek church music, you will easily realize the similarity of the ancient Roman chanting with that in use until today in the East.