While there may be discrepancy over terminology (the General Instruction speaks of Entrance Song, some liturgists, of Opening Song), it is clear what this first communal act of the assembly is intended to do.
It opens our celebration and fosters a unity that can be achieved solely through sung prayer.
Not only does it accompany the entrance of the priest and ministers into the assembly, but, more importantly, it helps the assembly to enter the presence of God and to enter more fully into the mysteries of the feast or season.
Today we recognize the great importance of the Opening Song in our liturgy, but it was not always so.
Let's examine the circular path that led to our present understanding of this ritual.
In the earliest days of the Church, the Breaking of the Bread, as the Mass was called, was held in homes or other private places.
No special ritual was needed to mark the beginning.
Greetings were undoubtedly exchanged among communities small enough that fellowship was rather spontaneous.
As the Church emerged from its period of persecution, Christians erected buildings for worship and the liturgy itself was expanded accordingly.
The faithful would gather in prayer and a chant signaled the beginning of the liturgy.
The first mention of this chant came during the pontificate of Celestine I (d.432).
This Introit, sung as the presider entered the assembly, took the form of an entire psalm sung by choir, with an antiphon for the people intercalated among the verses.
The chant was practical in that it accompanied the procession of the bishop or priest from the secretarium (sacristy) to the altar which was at the opposite end of the church, hence the name Introitus or "Entrance."
It was also liturgically apropos in that it set a theme for the celebration, by using the proper choice of psalm and antiphon.
In later centuries, as more prestige was awarded the pope and his bishops and priests, this entrance became quite elaborate.
Rev. William R. Bonniwell, in Interpreting the Sunday Mass, describes one such example:
The entrance of the ministers for Solemn Mss was an impressive ceremony.
The huge basilica would be crowded with the faithful who had flocked there from all parts of the city.
The clergy (bishops and priests) were already in their places in the sanctuary; the choir, placed near the altar, was waiting the signal to begin.
Meanwhile, the pope and his ministers were vesting in the sacristy which then was situated not near the altar but near the entrance of the church.
When the pontiff was ready, the cantor intoned an antiphon as the procession was leaving the vestibule and entering the church.
Leading the way were seven acolytes, each carrying a candlestick having seven lighted candles; next came a subdeacon, who swung a smoking censor of gold.
After him followed the various ministers of the Mass, and lastly the pontiff with the archdeacon to his right and a second deacon to his left.
As the procession moved down the length of the basilica to the sanctuary, the choir sang a Psalm of David and the people repeated, as a chorus after each verse, the antiphon which the cantor had intoned at the beginning.
Since this singing took place during the entrance of the ministers, it was called the Introitus, or "Entrance."
With the development of a highly trained choir, or schola, the people's part in the Introit gradually disappeared, leaving the psalm verses and antiphons to be sung by alternate sides of the choir.
Later times also witnessed a strong desire for expediency, hence the sacristy was moved to a position almost adjacent to the sanctuary.
Long entrance processions were no longer the rule and the singing of an entire psalm was abbreviated to an antiphon, a "token" psalm verse, the Gloria Patri, and the antiphon again.
The music for these Introits became quite ornate, the product of composers of florid chant later to be called "Gregorian."
This is the familiar format found in the pre-Vatican II liturgy of Pope St. Pius V.
With the reforms of the new liturgy came a return to the former practice of responsorial psalmody as the rule for the entrance rite.
Provision was made (significant especially in Anglo-Saxon regions of strong hymnic traditions) for the singing of metrical hymns as a replacement for this opening psalmody.
This has certainly become normative in the United States and other countries, although it must be noted that this option is not the ideal set down in the rubrics of the Roman Rite (the rite used by the vast majority of Western Catholicism).
Liturgical Music Today (U.S. Bishops' Committee on the Liturgy) states:
The entrance song serves to gather and unite the assembly and set the tone for the celebration as much as to conduct the ministers into the sanctuary.
While the responsorial form of singing is especially suitable for processions, the metrical hymn can also fulfill the function of the entrance song.
If, however, a metrical hymn with several verses is selected, its form should be respected.
The progression of text and music must be allowed to play out its course and achieve its purpose musically and poetically.
In other words, the hymn should not be ended indiscrimately at the end of the procession.
Here the bishops have unmistakably underscored the dignity of the Opening Song as sung prayer, in and of itself.