52 JC No 37

This is the last part of the preface to the Geneva Psalter (see here for the whole thing)

What is there now to do? It is to have songs not only honest, but also holy, which will be like spurs to incite us to pray to and praise God, and to meditate upon his works in order to love, fear, honor and glorify him. Moreover, that which St. Augustine has said is true, that no one is able to sing things worthy of God except that which he has received from him. Therefore, when we have looked thoroughly, and searched here and there, we shall not find better songs nor more fitting for the purpose, than the Psalms of David, which the Holy Spirit spoke and made through him. And moreover, when we sing them, we are certain that God puts in our mouths these, as if he himself were singing in us to exalt his glory. Wherefore Chrysostom exhorts, as well as the men, the women and the little children to accustom themselves to singing them, in order that this may be a sort of meditation to associate themselves with the company of the angels.
As for the rest, it is necessary to remember that which St. Paul hath said, the spiritual songs cannot be well sung save from the heart. But the heart requires the intelligence. And in that (says St. Augustine) lies the difference between the singing of men and that of the birds. For a linnet, a nightingale, a parrot may sing well; but it will be without understanding. But the unique gift of man is to sing knowing that which he sings. After the intelligence must follow the heart and the affection, a thing which is unable to be except if we have the hymn imprinted on our memory, in order never to cease from singing. For these reasons this present book, even for this cause, besides the rest which has been said, ought to be singular recommendation to each one who desires to enjoy himself honestly and according to God, for his own welfare and the profit of his neighbours: and so there is need of all of it being much recommended by me: seeing that it carries its value and its praise. But that the world may be so well advised, that in place of songs in part vain and frivolous, in part stupid and dull, in part foul and vile, and in consequence evil and harmful which it has used up to now, it may accustom itself hereafter to the singing of these divine and celestial hymns with the good king David.

Touching the melody, it has seemed best that it be moderated in the manner we have adopted to carry the weight and majesty appropriate to the subject, and even to be proper for singing in the Church, according to that which has been said.