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.
For it is a feature of this book that the Psalms which compose it are of many different sorts. Some such as 73, 78, 114, and 115, are narrative in form; some are hortatory, like 32, 97, and 103; some are prophetic, for example, 22, 45, 47, and 110; some, in whole or part, are prayers to God, as are 6, 16, 54, 102; some are confessions, notably the 51st, some denounce the wicked, like 14; while yet others, such as 8, 98, 117, 125, and many more, voice thanksgiving, praise, and jubilation, Psalm 66 alone of these having special reference to the Resurrection of the Lord.
It is possible for us, therefore, to find in the Psalter not only the reflection of our own soul’s state, together with precept and example for all possible conditions, but also a fit form of words wherewith to please the Lord on each of life’s occasions, words both of repentance and of thankfulness, so that we fall not into sin; for it is not for our actions only that we must give account before the judge, but also for our every idle word.
Suppose, then, for example, that you want to declare anyone to be blessed; you find the way to do it in Psalm 1, and likewise in 32, 41, 112, 119, and 128. If you want to rebuke the conspiracy of the Jews against the Saviour, you have Psalm 2. If you are persecuted by your own family and opposed by many, say Psalm 3; and when you would give thanks to God at your affliction’s end, sing 4 and 75 and 116.
When you see the wicked wanting to ensnare you and you wish your prayer to reach God’s ears, then wake up early and sing 5; and if you feel yourself beneath the cloud of His displeasure, you can say 6 and 38. If any plot against you, as did Ahithophel against David,, and someone tells you of it, sing Psalm 7, and put your trust in God Who will deliver you.
Contemplating humanity’s redemption and the Saviour’s universal grace, sing Psalm 8 to the Lord; and with this same Psalm or the 19th you may thank Him for the vintage. For victory over the enemy and the saving of created things, take not glory to yourself but, knowing that it is the Son of God Who has thus brought things to a happy issue, say to Him Psalm 9; and, if any wishes to alarm you, the 11th, still trusting in the Lord. When you see the boundless pride of many, and evil passing great, so that among men (so it seems) no holy thing remains, take refuge with the Lord and say Psalm 12.
And if this state of things be long drawn out, be not faint-hearted, as though God had forgotten you, but call upon Him with Psalm 27. Should you hear others blaspheme the providence of God, do not join with them in their profanity but intercede with God, using the 14th and the 53rd. And if, by way of contrast, you want to learn what sort of person is citizen of heaven’s kingdom, then sing Psalm 15.
When, again, you need to pray against your enemies and those who straiten you, Psalms 17, 86, 88, and 140 will all meet your need; and if you want to know how Moses prayed, you have the 90th.[Headed in the Septuagint, A Prayer of Moses, Man of God.]
When you have been delivered from these enemies and oppressors, then sing Psalm 18; and when you marvel at the order of creation and God’s good providence therein and at the holy precepts of the Law, 19 and 24 will voice your prayer; while 20 will give you words to comfort and to pray with others in distress.
When you yourself are fed and guided by the Lord and, seeing it, rejoice, the 23rd awaits you. Do enemies surround you? Then lift up your heart to God and say Psalm 25, and you will surely see the sinners put to rout. If they persist, their murderous intent unslaked, then let man’s judgement go and pray to God, the Only Righteous, that He alone will judge according unto right, using Psalms 26 and 35 and 43.
If your foes press yet harder and become a veritable host, that scorns you as not yet anointed, be not afraid, but sing again Psalm 27 [The title of Psalm 27 in the Greek is Of David, before he was annointed. The Christian reference is to chrismation, i.e., Confirmation, which was conferred as part of the same rite with Baptism in the early Church].
Pay no attention either to the weakness of your own humanity or to the brazenness of their attack, but cry unceasingly on God, using Psalm 28. And when you want the right way of approach to God in thankfulness, with spiritual understanding sing Psalm 29. And finally, when you dedicate your home, that is your soul in which you receive the Lord and the house of your senses, in which corporeally your spirit dwells, give thanks and say the 30th and, from the Gradual Psalms [Psalms 119 – 133] , the 127th.
Again, when you find yourself hated and persecuted by all your friends and kinsfolk because of your faith in Christ, do not despair on this account nor be afraid of them, but go apart and, looking to the future, sing Psalm 31. And when you see people baptized and ransomed from this evil world, be filled with wonder at the love of God for men, and in thanksgiving for them sing the 32nd.
And whenever a number of you want to sing together, being all good and upright men, then use the 33rd. When you have fallen among enemies but have escaped by wise refusal of their evil counsel, then also gather holy men together and sing with them the 34th.
And when you see how zealous are the lawless in their evil-doing, think not the evil is innate in them, as some false teachers say, but read Psalm 36 and you will see they are themselves the authors of their sin. And if you see these same wicked men trying, among other evils, to attack the weak and you wish to warn their victims to pay no heed to them, nor envy them, since they will soon be brought to nought, both to yourself and others say the 37th.
When, on the other hand, it is your own safety that is in question, by reason of the enemy’s attacks, and you wish to bestir yourself against him, say the 39th; and if, when he attacks, you then endure afflictions, and wish to learn the value of endurance, sing Psalm 40.
When you see people in poverty, obliged to beg their bread, and you want to show them pity, you can applaud those who have already helped them and incite others to like works of mercy by using 41. Then again, if you are aflame with longing for God, be not disturbed at the reviling of your enemies but, knowing the immortal fruit that such desire shall bear, comfort your soul and ease your pains with hope in God, and say the 42nd.
When you wish to recall in detail the loving-kindnesses which God showed to the fathers, both in their exodus from Egypt and in the wilderness, and to reflect how good God is and how ungrateful are men, you have the 44th, the 78th, the 89th, the 105th, 106th, 107th, and also the 114th and 115th. And the 46th will supply your need when after deliverance from afflictions you flee to God, and want to give Him thanks and tell of all His loving mercy shown towards yourself.
But suppose now that you have sinned and, having been put to confusion, are repenting and begging for forgiveness, then you have the words of confession and repentance in Psalm 51. Or you have been slandered, perhaps, before an evil king, and you see the slanderer boasting of his deed: then go away and say Psalm 52. And when they persecute and slander you, as did the Ziphites and the strangers to King David [1 Kings 23:13ff], be not disturbed but with full confidence in God sing praise to Him, using Psalms 54 and 56.
If still the persecution follows hard on you, and he who seeks your life enters (though he knows it not) the very cave in which you hide [1 Kings 24:3], still you must not fear; for even in such extremity as this you have encouragement in Psalm 57 and also in the 142nd.
The plotter, it may be, gives orders that a watch be kept over your house, and yet you manage to escape; give thanks to God, then, and let Psalm 59 be written on your heart, as on a pillar, as a memorial of your deliverance. And if not only your enemies cast you in the teeth but those also whom you thought to be your friends reproach and slander you and hurt you sorely for a time, you can still call upon God for help, using Psalm 55.
Against hypocrites and those who glory in appearances, say for their reproach the 58th. But against those whose enmity is such that they would even take away your life, you must simply oppose your own obedience to the Lord, having no fear at all but all the more submitting to His will as they grow fiercer in their rage, and your form of words for this will be the 62nd Psalm.
Should persecution drive you to the desert, fear not as though you were alone in it, for God is with you, and there at daybreak you may sing to Him the 63rd. And if even there the fear of foes and their unceasing plots pursues you, be they never so many or so insistent in their search for you, still you must not yield; for the toy arrows of a child will be enough to wound them, while Psalms 64, 65, 70, and 71 are on your lips.
The 65th Psalm will meet your need, whenever you desire to sing praise to God: and if you want to teach any one about the Resurrection, sing the 66th. When asking mercy from the Lord, praise Him with the 67th. When you see wicked men enjoying prosperity and peace and good men in sore trouble, be not offended or disturbed at it but say Psalm 73.
When God is angry with His people, you have wise words of comfort in Psalm 74. When you have occasion to testify concerning God, 9, 71, 75, 92, 105 to 108, 111, 118, 126, 136, and 138 all fit the case; and Psalm 76, when used intelligently, provides you with an answer for the heathen and the heretics, showing that the knowledge of God is not with them at all, but only in the Church.
And when the enemy takes possession of your place of refuge, even though sorely harassed and afflicted, do not despair but pray: and when your crying has been heard, give thanks to God, using Psalm 77. And if they have profaned the house of God and slain the saints, throwing their bodies to the birds of prey, do not be crushed or frightened at such cruelty, but, suffering with those that suffer it, plead you for them with God, using Psalm 79.
Psalms 81 and 95 are suitable if you want to sing on a festival, together with other servants of the Lord; and when the enemy once more muster round you, threatening God’s House and joining forces against His holy ones, do not you be frightened of either their numbers or their strength, for you have a very anchor of hope available in Psalm 83.
If, moreover, you behold the House of God and His eternal dwelling, and have a longing for them, as the Apostle had, then say the 84th; and when at length their anger is abated and you are free again, voice your thanksgiving in the 85th and in the 116th. To see the difference between the Church and schism and to confound schismatics, you can say 87. To encourage yourself and others in the fear of God and to show how fearless is the soul that hopes in Him, say 91.
Do you want to give thanks on the Lord’s Day? Then say the 24th; if on a Monday, then the 95th; and if on a Friday, your words of praise are in the 93rd, for it was when the Crucifixion was accomplished that the House of God was built, for all the enemy attempted to prevent it, so it is fitting we should sing on Friday a song of victory, such as that Psalm is.
Psalm 96 is apt, if God’s House has been captured and destroyed and then re-built; and when the land has rest from war and peace returns, sing that The Lord is King in 97. You want to sing on Wednesday? The Psalm then is 94; for it was on the fourth day from the Sabbath [This Psalm is headed in the Septuagint, A Psalm of David for the fourth day from do Sabbath] that the Lord through His betrayal entered on His Passion, by which He should redeem us and by the which He triumphed gloriously.
So when you read in the Gospel how on the Wednesday the jews took counsel against the Lord, seeing Him thus boldly challenging the devil on our behalf, sing the words of this Psalm 94. And again, when you see the providence and power of God in all things and want to instruct others in His faith and obedience, get them first to say the 100th Psalm. And when you have yourself experienced His power in judgement (for always His justice is tempered by His mercy) the next Psalm  will express your need.
If through the weakness of your nature and the strain of life you find yourself at times downcast and poor, sing for your consolation Psalm 102, and use the two that follow it [103, 104] to lift your heart in thankful praise to God, as in and through all circumstances we should always do. Psalms 105, 107, 113, 117, 135, and 146 to 150 not only show the reasons why God should be praised, but tell you how to do it.
Have you faith, as the Lord bade, and believe in the prayers you utter? Then say the 116th Psalm, from the tenth verse on. You feel that, like the Apostle, you can now press forward, forgetting all the things that lie behind? [Phil 3:14] Then you have the fifteen Gradual Psalms [Psalms 119 – 133] for every step of your advance.
Another time, perhaps, you find you have been led astray by others’ arguments-well, then, the moment you perceive it, stop your sinning, sit down and weep, as they did of old by Babylon’s waters, using the words of Psalm 137. Since it is precisely by being tempted that one’s worth is proved, Psalm 139 will meet your need when you thank God for testing safely past.
And if the enemy once more gets hold of you and you desire to be free, then say 140. For prayer and supplication, sing Psalms 5, 141 to 143, and 146. Has some Goliath risen up against the people and yourself? Fear not, but trust in God, as David did, and sing his words in Psalm 144. Then, marvelling at God’s kindnesses to everyone and mindful of His goodness to yourself and all, praise Him, again in David’s words, with Psalm 105.
You want to sing to Him? Use 96 and 98. If, weak as you are, you yet are chosen for some position of authority among the brethren, you must not be puffed up as though. you were superior to them, but rather glorify the Lord Who chose you and sing Psalm 151, which is especially the Psalm of David. And for Psalms in praise of God, having some of them the title Alleluya, you have all these, 105 to 107, 111 to 118, 135, 136, 146, 147, 148, 149, and 150.
If, again, you want to sing Psalms that speak especially about the Saviour, you will find something in almost all of them; but 45 and 110 to relate particularly to His Divine Begetting from the Father and His coming in the flesh, while 22 and 69 foretell the holy cross, the grievous plots He bore and how great things He suffered for our sakes.
The 3rd and 109th also display the snares and malice of the Jews and how Iscariot betrayed Him; 21, 50, and 72 all set Him forth as judge and foretell His Second Coming in the flesh to us; they also show the Gentiles’ call. The 16th shows His resurrection from the dead, in flesh, the 24th and 47th His ascension into heaven. And in the four Psalms 93, 96, 98, and 99, all the benefits deriving to us from the Saviour’s Passion are set forth together.
Taken from Athanasius’ letter to Marcellinus found here.