From my Master’s thesis:
The basis of iconography is the divine Incarnation of God the Word. When God the Word became Man He gave a visible image to the invisible God and thus facilitated the existence of icons of the God-Man: “in the icon of Christ the person of Christ is made visible according to His human nature, just as He became visible and historical in His incarnation of the flesh.”
God the Word became circumscribed in His historical incarnation and thus the iconographer can now circumscribe Him in icons: “But if He assumed humanity in truth, as we confess, then the hypostasis of Christ is circumscribable: not according to its divinity, which no one has ever beheld, but according to the humanity which is contemplated in an individual manner in it (10)”. However, this does not mean that the iconographer merely depicts the human nature of Christ, rather he depicts Christ’s person (hypostasis). That is, he depicts His full humanity and His full divinity as they are contained in His divine person: “neither the divine nor the human nature alone is depicted, but the hypostasis of Christ with the particular characteristics which define His human nature, that which the icons of Christ present is the person of the God-Man, the person of the whole God and of the whole man and it is understood and exists with His two natures.” Wherefore, the iconographer ought to take care when painting icons, for he is clothing – in line and colour – the invisible God according to His visible image, the God-Man Jesus Christ.
This is the foundation not only of icons of the God-Man, but of His saints as well: “The embodiment of God in Christ, the true humanity of Christ which can be seen and touched, is precisely the basis and fount of the icon. If there had been no Incarnation, no descent of God to earth, there could be no icons of God. Similarly, if there had been no Ascension of man into heaven in Christ, and if there had been no Pentecost, which is the descent of God into man, there could be no saints and therefore no icons of humans.”
Since saints are dwelling places of the Holy Spirit, when they are painted in icons it is not merely their human nature that is depicted but their whole person which participates in the uncreated grace of God and thus once again, the iconographer puts into colour and line what is invisible, “I cherish… everything connected to God’s name, not on their own account but because they show forth the divine power… I venerate and worship angels and men, and all matter participating in divine power and ministering to our salvation through it”.