John of Damascus

But since some find fault with us for worshipping and honouring the image of our Saviour and that of our Lady, and those, too, of the rest of the saints and servants of Christ, let them remember that in the beginning God created man after His own image. On what grounds, then, do we shew reference to each other unless because we are made after God's image? For as Basil (the Great, c. 330-379), that much-versed expounder of divine things, says, the honour given to the image passes over to the prototype. Now a prototype is that which is imaged, from that which the derivative is obtained.
But besides this who can make an imitation of the invisible, incorporeal, uncircumscribed, formless God? Therefore to give form to the Deity is the height of folly and impiety. And hence it is that in the Old Testament the use of images was not uncommon. But after God in His bowels of pity became in truth man for our salvation, not as He was seen by Abraham in the semblance of a man, nor as He was seen by the prophets, but in being truly man, and after He lived upon the earth and dwelt among men, worked miracles, suffered, was crucified, rose again and was taken back to Heaven, since all these things actually took place and were seen by men, they were written for the remembrance and instruction of us who were not alive at that time in order that though we saw not, we may still, hearing and believing, obtain the blessing of the Lord. But seeing that not every one has a knowledge of letters nor time for reading, the Fathers gave their sanction to depicting these events on images as being acts of great heroism, in order that they should form a concise memorial of them. Often, doubtless, when we have not the Lord's passion in mind and see the image of Christ's crucifixion, His saving passion is brought back to remembrance, and we fall down and worship not the material but that which is imaged: just as we do not worship the material of which the Gospels are made, nor the material of the Cross, but that which these typify. For wherein does the cross, that typifies the Lord, differ from a cross that does not do so? it is just the same also in the case of the Mother of the Lord. For the honour which we give to her is referred to Him Who was made of her incarnate. And similarly also the brave acts of holy men stir us up to be brave and to emulate and imitate their valor and to glorify God. For as we said, the honour that is given to the best of fellow-servants is a proof of good-will towards our common Lady, and the honour rendered to the image passes over to the prototype. But this is an unwritten tradition, just as is also the worshipping towards the East and the worship of the Cross, and very many other similar things.
Moreover that the Apostles handed down much that was unwritten, Paul, the Apostle of the Gentiles, tells us in these words: "Therefore, brethren, stand fast and bold the traditions which ye have been taught of us, whether by word or by epistle." And to the Corinthians he writes, "Now I praise you, brethren, that ye remember me in all things, and keep the traditions as I have delivered them to you."