From a letter written by St. Athanasius:
The Book of Psalms has a certain grace of its own. For in addition to the other things in which it enjoys fellowship with the other books of the Bible, it possesses this marvel – that it contains all the emotions of each soul and their various changes. Thus, through hearing, it teaches us not only not to disregard passion, but also how to heal passion through speaking and acting.
There is also this astonishing thing in the Psalms. After the prophecies about the Saviour and the nations, he who recites the Psalms is uttering the rest as his own words, and each sings them as if they were written concerning him. And it seems to me that these words become like a mirror to the person singing them, so that he might perceive himself and the emotions of his soul. For in fact he who hears the cantor receives the song that is recited as being about him, and either, when he is convicted by his conscience, he will repent, or hearing of the hope that resides in God, and how this kind of grace exists for him, he exults and begins to give thanks to God. Therefore, when someone sings the third psalm, recognising his own tribulations, he considers the words in the psalm to be his own. And then when someone sings the fiftieth, he is speaking the proper words of his own repentance. If the point needs to be put more forcefully, let us say that the entire Holy Scripture is a teacher of virtue and the truths of faith, while the Book of Psalms presents the perfect image for the soul’s course of life.
Such, then, is the help for mankind to be gained from the Book of Psalms. It is important not to pass over the question of why words of this kind are chanted with melodies. For some of the simple among us, although they believe indeed that the phrases are divinely inspired, imagine on account of the sweetness of sound that the psalms are sung for the sake of the ear’s delight alone. But this is not so. For Scripture did not just seek pleasant things; even these have been fashioned for the benefit of the soul. This is because it is fitting for Divine Scripture to praise God not in compressed speech alone, but also in the voice that is richly broadened.
Some things are said in close sequence; such as the Law and the Prophets and the histories, along with the New Testament. But on the other hand, some things are expressed more broadly, such as the psalms, odes, and songs, so that men will love God with their whole strength and power. Just as the harmony that unites flutes effects a single sound, so also, seeing that different movements appear in the soul, reason intends man to be neither discordant in himself, nor to be at variance with himself.
Reason intends the soul possessing the mind of Christ to use this as a leader, and by it to be a master of its passions. A man then becomes a stringed instrument and, devoting himself completely to the Spirit, obeys the mind of Christ, which acts like a plectrum in all his members and emotions, thus enabling him to serve the will of God. The harmonious singing of the Psalms is a figure and type of such order and tranquillity. For just as we discover the ideas of the soul and communicate them through the words we put forth, so also the Lord, wishing the melody of the words to be a symbol of the spiritual harmony in a soul, has ordered that the odes be chanted tunefully, and the Psalms recited with song.
St Athanasius, Letter to Marcellinus, 10-12, 14, 27-29; CWS (1980) tr. Gregg.