Gregorian Chant developed naturally out of the music heard in the ancient Jewish Temple in Jerusalem. Even into apostolic times there were still Levitical priests singing music to accompany the liturgical sacrifice. Their "hymnal" was the book of psalms; look through the psalms, and you'll notice that many of them include musical instructions for the leader, singers, or instrumentalists.
The first Jewish Christians were the inheritors of this tradition; we read in Paul's letters of the singing of the early Christian community, and in the Book of Revelation of the singing in heaven. A modern scholar sums it up this way:
"[T]he first followers of Christ sang the customary Jewish Psalms; and they supplemented these with songs of specifically Christian content, addressing them to one another or directly to God; and they looked forward to singing God's praises eternally in the hereafter." — Ben Whitworth, Music in the Liturgy
Over the first several hundred years of Christianity, this body of chant blossomed and spread. St. Ephraim of Syria and St. Ambrose of Milan were both important composers of liturgical music, as well as St. Thomas Aquinas centuries later, who created the beautiful Eucharistic chants for the feast of Corpus Christi. This body of chant bears the name "Gregorian" because of either Pope St. Gregory I ("the Great") or Pope St. Gregory II, although the precise reason why is lost to history.
Some time around the 13th-century singers began singing chants at more than one pitch simultaneously, and then they began to ornament the melody and alter the rhythm, and polyphony was born. With the creation of modern music notation by the monk Guido d'Arezzo, composers began writing down ever more complex musical settings, and the apex of polyphony is often said to be the late 16th-century — the High Renaissance — when composers such as Palestrina (Italy), Byrd (England), Victoria (Spain), and Lassus (the low countries) wrote music of astonishing beauty and equally amazing prodigiousness.
Even Vatican II — that is, the Second Vatican Council — recognized the importance of this music, profound in its beauty and uniquely suited for Mass, writing in Sacrosanctum Concilium, "The musical tradition of the universal Church is a treasure of inestimable value, greater even than that of any other art." [paragraph 112] The document states quite emphatically the need for Gregorian chant in the Mass, while also allowing the use of other musical forms, "especially polyphony." [paragraph 116]
We encourage you to introduce this musical treasury into your parish and even your home. As the venerable Latin phrase has it, qui bene cantat bis orat — he who sings well prays twice! Let us help you double your "prayer power" by bringing beautiful, authentic, Catholic sacred music into liturgy, home, and heart.