Briefly, then, if indeed any more is needed to drive home the point, the whole divine Scripture is the teacher of virtue and true faith, but the Psalter gives a picture of the spiritual life. And, just as one who draws near to an earthly king observes the formalities in regard to dress and bearing and the correct forms of words lest, transgressing in these matters, he be deemed a boor, so he who seeks to live the good life and learn about the Saviour's conduct in the body is by the reading of this holy book first put in mind of his own soul's condition and then supplied with fit words for a suppliant's use.- Athanasius, Letter to Marcellinus
Isn't it nice how Athanasius manages to work in a reference to the unique teaching role of Scriptures in his letter? It is hard for him to talk about his sole rule of faith and consequently we see it bursting forth, even though the subject matter at hand does not really require it. Isn't it also nice how, in effect, he works in the concept of the regulative principle, reminding his readers that we should learn from the Scriptures how God wishes to be worshiped? After all, the regulative principle of worship is the other side of the coin of Sola Scriptura.
Yet Athanasius is not content simply with praising the Psalms. Athanasius seems to have been one who would not have been fond of Watts' attempted improvements of the Psalms, just as doubtless he would have been opposed to the modern tendency to derogate the Psalms to a relatively minor role in worship:
There is, however, one word of warning needed. No one must allow himself to be persuaded, by any arguments what-ever, to decorate the Psalms with extraneous matter or make alterations in their order or change the words them-selves. They must be sung and chanted in entire simplicity, just as they are written, so that the holy men who gave them to us, recognizing their own words, may pray with us, yes and even more that the Spirit, Who spoke by the saints, recognizing the selfsame words that He inspired, may join us in them too. For as the saints' lives are lovelier than any others, so too their words are better than ever ours can be, and of much more avail, provided only they be uttered from a righteous heart. For with these words they themselves pleased God, and in uttering them, as the Apostle says, they subdued kingdoms, they wrought righteousness, they obtained promises, they stopped the mouths of lions, quenched the power of fire, escaped the edge of the sword, from weakness were made strong, waxed mighty in war, turned to flight armies of aliens, women received their dead by resurrection. [Heb 11:33-36]- Athanasius, Letter to Marcellinus
For those interested in reading more, including some very exact suggestions as to which Psalms are particularly fitting for various occasions, see the following link (link).
To the Glory of God, whose mercies endure forever and whose grace never fails (Psalm 136),