The Psalms are truly a reflection upon our souls. In times of deepest need and greatest happiness they can mirror the inward thoughts and feelings of us all as Calvin and Luther explicate below.
There is not an emotion of which anyone can be conscious that is not here represented as in a mirror. The Holy Spirit has here drawn to the life all the griefs, sorrows, fears, doubts, hopes, cares, perplexities, in short, all the distracting emotions with which the minds of men are often agitated. The other parts of Scripture contain the commandments which God enjoined his servants to announce to us, but here the prophets themselves seeing they are exhibited to us as speaking to God and laying open all their innermost thoughts and affections call, or rather draw, each of us to the examination of himself in particular in order that none of the many infirmities to which we are subject and of the many vices with which we abound may remain concealed. It is certainly a rare and singular advantage when all lurking places are discovered and the heart is brought into the light, purged from that most painful infection, hypocrisy. In short, as calling
upon God is one of the principle means of securing our safety and as a better and more unerring
rule for guiding us as in this exercise cannot be found elsewhere than in the Psalms, it follows
that in proportion to the proficiency which a man shall have attained in understanding them will
be the knowledge of his most important part of celestial doctrine. Genuine and earnest prayer
proceeds first from a sense of our need and next from faith in the promises of God. It is by
perusing these inspired compositions that men will be most effectually awakened to a sense of
their maladies and, at the same time, instructed in seeking remedies for their cure. In a word,
whatever may serve to encourage us when we are about to pray to God is taught us in this book.
- Taken from Calvin's preface to his commentary on the Psalms.
...Wouldest thou see the Holy Christian Church portrayed in living form and color, as it were in miniature? Open the Psalter. Thus thou shalt have before thee a fine, bright, spotless mirror, that will shew thee what kind of thing Christianity is. Yea, thou shalt therein find thine own self, and the right "know thyself"; God himself also and all his creatures. - From the preface of Martin Luther's Commentary on the First Twenty-Two Psalms.