It is a well-known fact that for the early centuries of Christianity, Koine Greek was the predominant language among believers, reflecting its status in the Empire as a lingua franca and the hold it has on the eastern areas (where Christianity originated). The same held true for the city of Rome itself (initially to the large number of Greek slaves in Roman households).
However, by the 4th century AD, Greek no longer held such dominance over Latin in the western Church, arts and sciences as it had previously, resulting to a great extent from the growth of the western provinces. As centuries went on, popular knowledge of Greek disappeared, save for the scraps which the liturgies of the West preserved (one of which is the familiar Kyrie Eleison). "Latin was enough of a problem by itself: it was no longer anyone's native language, but it was nevertheless indispensable as the language of the liturgy, political administration, scholarship, and most arts, and almost all energy expended on language learning was concentrated on Latin. Greek could at best be one's "second foreign language"-and thus only a very few medieval Westerners acquired the ability to understand a Greek text with unfamiliar content." (Walter Berschin, "Valuation and Knowledge of Greek")
Due to the scholarly interest in Greek, there circulated a fad of using the Greek language for the ordinary as well as the propers of the Mass at high feasts in a number of places during the Carolingian period (ca. 9th-11th centuries AD), which in turn survived in a few locales such as the Royal Abbey of Saint-Denis (which retained this tradition well into the French Revolution). These texts - usually showing the Greek in transliterated Latin characters - are collectively known as the Missa Graeca, the "Greek Mass". For example, three out of many variants of transliterating the Agnus Dei (along with the actual Greek version):
1: O Amnos tutheu, o eronthas amarthias tuchosmu, eleyson imas.
2: Ao amnos tutheu, oerronan tin amartias tu cosmu, eleison imas.
3: Oamnos tu theu, oerontas amartias tu kosmu, eleison imas.
ὁ ἀμνὸς τοῦ Θεοῦ, ὁ αἴρων τὰς ἁμαρτίας τοῦ κόσμου, ἐλέησον ἡμᾶς.
(O amnos tou Theou, o airon tas amartias tou kosmou, eleison imas).
Here, meanwhile is the Sanctus and the Gloria from an 9th-century missal from St. Denis (BN lat. 2290):
Doxa en ypsistis Theo ke epi gis irini en antropis eudokia enumen se eulogumen se proskunumen se doxologumen se eucharistumen se dia tin megalin su doxan Kyrrie basileu ep uranie thee Pater pentocrator kyrrie yie monogeni Iisu Christe ke Agion Pneuma. Kyrrie o Theos o amnos tutheu o yios tu Patros o eron tin amartian tu kosmu eleison imas oerontas amartias tu kosmu pros de ke tin deisin imon kathimenos en dexia tu Patros eleison imas oti sy i monos agios sy iomonos Kyrrios sy i monos ypsistos Iisus Christos syn Agion Pneumati is doxan Theu Patrosa. Amin.
Agios Agios Agios Kyrios pliris
ouranos ke igitis doxis
osanna entys ypsistys
eulogemenos o ercomenos
en onomati Kyrriu
osanna entys ypsistis.