Were one to open the Roman Missal at the first page, finding there the Mass of the First Sunday of Advent, the very first element proper to that Mass, and to all others, is the Introit.
The Introit is composed of an antiphon, a verse taken from the psalm corresponding to the antiphon or, occasionally, from another, the Gloria Patri, and the repetition of the antiphon.
The Introit as presented in the Roman Missal appears in a somewhat truncated form, though all the essential elements — antiphon, psalmody, and doxology — are present. Until about the eighth century the entire psalm would have been chanted, or at least the greater part of it, with the antiphon repeated after every verse, and this until the celebrant reached the altar, at which point the cantors would intone the Gloria Patri, and after the final repetition of the antiphon, end the Introit.
The purpose of the Introit in the tradition of the Roman Rite is not didactic; it is contemplative. The Introit ushers the soul into the mystery of the day not by explaining it, but by opening the Mass with a word uttered from above. The text of the Introit signifies that, in every celebration, the initiative is divine, not human; it is a word received that quickens the Church–at–Prayer, and awakens a response within her.