Gregorian chant
The sorts of songs, which Catholics later sang in the Roman Rite, started to appear in the 3rd century. The Apostolic Tradition, a book about Christian traditions in early Rome, says that Christians sang "Alleluia" in early holy meals called "agape feasts." Chants of the Office started in the early 4th century, when holy men in the deserts started the tradition of singing all 150 psalms every week. Sometime around 375, Christians in Eastern Europe started singing religious songs back and forth between two groups; in 386, St. Ambrose brought this tradition to Western Europe. Singing back and forth is called "antiphonal" singing.