According to the author, the concept of plain chants was

Directions: Read the following passage and answer the question.
The modern entertainment called opera is a child of the Roman Catholic Church. What might be described as operatic tendencies in the music of worship date further back than the foundation of Christianity. The Egyptians were accustomed to sing "jubilations" to their gods, and the Greeks caroled on vowels in honor of their deities. From these practices descended into the musical part of the earliest Christian worship a certain rhapsodic and exalted style of delivery.
That this element should have disappeared for a considerable time from the church music is not at all remarkable, for in the first steps toward regulating the liturgy, simplification was a prime requisite. Thus in the centuries before Gregory, the plain chant gained complete ascendancy in the church and under him it acquired a systematization which had in it the elements of permanency.
Yet it was through the adaptation of this very chant to the delineation of episodes in religious history that the path to the opera was opened. As ceremonials became more and more elaborate, they approached more and more closely the ground on which the ancient dramatic dance rested, and it was not long before they themselves acquired a distinctly dramatic character. It is at this point that the liturgical ancestry of the opera becomes quite manifest. The dance itself, at first an attempt to delineate dramatically by means of measured movement was not without its place in the early church. At one time Christian priests executed solemn dances before their altars just as their Greek predecessors had done. But in the course of time the dance became generally practiced by the congregation and this gave rise to abuses. The authorities of the church abandoned it. But the feeling for it lingered, and in after years issued in the employment of the procession. When the procession left the sanctuary and displayed itself in the open air, something of the nature of the dance returned to it and its development into a dramatic spectacle was not difficult.
Coussemaker finds that lyric drama had in its inception two chief varieties, namely, the secular drama, and the religious or liturgical drama. “Each of these dramas,” he says, "had its own particular subject matter, character, charms and style.
The liturgical drama, which was chronologically the first of the two forms, originated in the ceremonies of the Christian church, in the strong dramatic element which inheres in the mass, the Christmas fêtes, and the Epiphany. These are all scenes in the drama of the sacrifice of the Redeemer, and it required but small progress to develop them into real dramatic performances, designed for the instruction of a people which as yet had no literature.
Out of these ceremonies, then, grew the liturgical drama. It is certain that the educational drama of the church continued in the state of its infancy for several centuries. Several interesting manuscripts in great libraries attest the consideration accorded to it at a period much later than that of which we have been speaking.