God became man so that man might become God by grace. Or as St John of Damascus put it:
I do not worship the creation rather than the Creator, but I worship the one who became a creature, who was formed as I was, who clothed Himself in creation without debasement or departing from His divinity, that He might raise my nature in glory and make it a partaker of His divine nature.
This explains why we have icons of holy people as well as of Christ. In the saint we see Christ shining forth. We worship God alone, but we venerate and honour all those people and things through which God comes to us.
Deification, or transfiguration as it may be termed, also explains the characteristic style of icons. The way an icon is painted suggests a world shining with the glory of God. It is not just what is depicted that is significant about the icon tradition, but how this is depicted. It is possible to depict a holy person in a profane way, omitting to suggest their transfigured state. Conversely, one can depict a mundane object in a sacred way, showing it in its paradisiacal state.
Deification is the norm that God intended for man, and so a naturalistic portrait, as wonderful and sympathetic as it might be in a painting, is not actually depicting man in his full and ‘natural’ supra-natural state.
Aidan Hart, Techniques of Icon and Wall Painting, 3.