Psalms are song by emperors; the common people rejoice in them. Each man does his utmost in singing what will be a blessing to all. Psalms are sung in the home and rehearsed on the streets. The psalm is learned without labor and remembered with delight. Psalmody unites those who disagree, makes friends of those at odds, and brings together those who are out of charity with one another. Who could retain a grievance against the man whom he had joined in singing before God? The singing of praise is the very bond of unity, when the whole people join in a single act of song.
—Ambrose on Psalm 1, Exposition 9, trans. Erik Routley in The Church and Music (London: Duckworth, 1950), 129.
Chapter VII.—Of the Church Hymns Instituted at Milan; Of the Ambrosian Persecution Raised by Justina; And of the Discovery of the Bodies of Two Martyrs.
“At this time it was instituted that, after the manner of the Eastern Church, hymns and psalms should be sung, lest the people should pine away in the tediousness of sorrow; which custom, retained from then till now, is imitated by many, yea, by almost all of Thy congregations throughout the rest of the world.”
– “Augustine on Hymn Singing in Milan during the time of Ambrose,” Confessions, 9. 7. 15