Music is of two kinds (as are the other arts also)—secular and ecclesiastical. Each of these has been developed by different feelings and different states of the soul. Secular music expresses worldly (i.e., carnal) feelings and desires. Although these feelings may be very refined (romantic, sentimental, idealistic, etc.), they do not cease being carnal. Nevertheless, many people believe that these feelings are spiritual. However, spiritual feelings are expressed only by ecclesiastical music. Only ecclesiastical music can truly express the secret movements of the heart, which are entirely different from those inspired and developed by secular music. That is, it expresses contrition, humility, suffering and godly grief, which, as Paul says, "worketh repentance to salvation."  Ecclesiastical music can also evoke feelings of praise, thanksgiving, and holy enthusiasm. Secular music, on the other hand—even the purest—expresses carnal emotions, even when it is inspired by suffering and affliction. This type of suffering, Paul calls "worldly grief," which "worketh death." 
Thus two kinds of music were formed, the secular, which arouses
emotion—any kind of emotion—and ecclesiastical music, which evokes contrition. St. John Chrysostom strongly condemns the attempts that were made by some of his contemporaries to introduce into the Church secular music, the music of the theatre and the mimes.
Only the arts which were developed by devout motives since the early years of Christianity have given expression to the spiritual essence of the religion. These alone can be called liturgical, that is, spiritual, in the sense that religion gives to the term spiritual. The "spiritual odes" of which Paul speaks  were works of such art. All the liturgical arts express the same thing: architecture, hymnody, iconography, embroidery, and even writing, the manner of walking, and in general the movements and gestures of the priests, the chiming of the bells, and so forth.
That these arts are truly of unique spirituality has been realized by many non-Orthodox, especially clergymen, whose sense-organs have been exposed, from youth on, to formative influences different from those in which Orthodox Christians have been brought up. Nevertheless, they confess that our icons and psalmody evoke in them contrition-of course, when executed by inspired and pious artists.
Thus, the value of the liturgical arts is not merely conventional, but real, extending beyond the limited conceptions that are due to nurture, habit, and taste, since even persons who are not of the Orthodox faith recognize that the arts of the Orthodox Church reflect the spirit of the Gospels and for this reason lift the soul above the earthly realm. And how could it be otherwise, inasmuch as these arts have been developed by sanctified hearts, which felt deeply the liturgical element in speech and music? Liturgical music is the natural musical garb of liturgical speech. Both sprang up together; they are one and the same thing. Essence and expression here have an absolute correspondence, even more exact than that of an object and its reflection in a mirror, for the objects of which we speak here belong to the spiritual realm. The profound and apocalyptic spirit of Christian religion and its mysteries could not be expressed faithfully and worthily except by these arts, which are called liturgical and spiritual, and which were developed by that same profound spirit. Only this music, and none other, uniquely expresses the spirit of our religion, because only this music has an absolute and most exact correspondence with it. This is testified to, I repeat, by certain men whose spiritual upbringing, religious training, phyletic and other heritage have no relation to that of the Orthodox. "The Spirit bloweth where it listeth,"  and is transmitted to souls by means of sounds which the same Spirit formed, by illuminating the souls of the holy writers of hymns.
The Fathers of the Church ordained that Christians use the voice alone in execution of hymns, chanting as did our Lord Himself and His disciples. St. John Chrysostom says: "Our Savior chanted hymns just as we do." The Apostolic Constitutions forbid the use of musical instruments in the church. From the time of the Apostles, psalmody was monophonic, or homophonic, as it is to this day in our churches [in Greece].
The Western Church, in order to gratify people and flatter their tastes, put instruments inside the churches, disobeying what was ordained by the Fathers. They did this because they had no idea what liturgical music was and what secular music was, just as they did not know the difference between liturgical painting and secular painting. But the Byzantines distinguished the one from the other, and this shows how much more spiritual they were in comparison with the Westerners and how much more truly they experienced the spirit of Christianity. Byzantine music is, in comparison with the music of the West, exactly as Orthodox iconography is in comparison with the religious painting of the West.
How divine, indeed, is the psalmody of the Orthodox Church! It seems sweeter and sweeter each year to the Christian—a new wine that fills the heart with joy and makes it soar to the ethereal region of immortal life.
Byzantine music is peaceful, sad but consoling, enthusiastic but reserved, humble but heroic, simple but profound. It has the same spiritual essence as the Gospels, the hymns, the psalms, the books of the lives of the saints, and the iconography of Byzantium. That is why Byzantine music is monotonous for one to whom the Gospels are monotonous, naive for one to whom the Gospels are naive, circumscribed for one to whom the Gospels are circumscribed, mournful for one to whom the Gospels are mournful, antiquated for one to whom the Gospels are antiquated. But it is joyful for one to whom the Gospels are joyful, filled with compunction for one to whom the Gospels are filled with compunction, enthusiastic but humble for one to whom the Gospels, are enthusiastic but humble, and peaceful for one who experiences the peace of Christ.
Byzantine art is spiritual, and it is necessary that a man have spiritual depth in order to understand its mystical treasures. Byzantine music expresses "gladdening sorrow,"  that is, that spiritual fragrance which only the spiritual senses are capable of experiencing. Its melody is not unholy, ostentatious, despondent, shallow, tasteless, or aimless; it is meek, humble, sweet with a certain bittersweetness, and full of contrition and mercy. It bestows an unwaning spiritual glory upon souls that have become worthy of the eternal mysteries and the compassion of God. It expresses thanksgiving; it causes the flow of tears of gratitude and spiritual joy. This music is the warmest, the most direct, and the most concise expression of the religious feeling of faithful Orthodox people.
Photios Kontoglou of blessed memory (1895-1965) played a major role in the glorious return of traditional Byzantine iconography to the Greek Orthodox world in the twentieth century. He was also an accomplished chanter and a spiritual writer who inspired countless souls to embrace the unadultered traditions of the Orthodox faith. This epilogue consists of selections from his writings translated in the book Byzantine Sacred Art by Dr. Constantine Cavarnos, who was one of his disciples.
 II Cor. 7:10
 Vid. Eph. 5:19 and Col. 3:16
 Jn. 3:8
 Vid. The Ladder, Step 7:9 (Migne, Patrologia Graeca, vol. 88, col. 804B)