The eye-witnesses and ministers of the word handed down the teaching of the Church, not only by writing, but also by unwritten tradition. Whence comes our knowledge of the sacred spot, Mount Calvary, of the holy sepulchre? Has it not been handed down to us from father to son? It is written that our Lord was crucified on Calvary, and buried in the tomb which Joseph hewed out of the rock, but it is unwritten tradition that teaches us we are adoring the right places, and many other things of the same kind. Why do we believe in three baptisms, that is, in three immersions? Why do we adore the Cross? Is it not through tradition? Therefore the holy apostle says: “Brethren, stand fast; and hold the traditions which you have learned, whether by word, or by our epistle.” (II Thess. 2.15) Many things, therefore, being handed down to the Church by unwritten tradition and kept up to the present day, why do you speak slightingly of images?
I am not to be persuaded that the Church is set in order by imperial edicts, but by patristic traditions, written and unwritten. As the written Gospel has been preached in the whole world, so has it been an unwritten tradition in the whole world to represent in image Christ, the incarnate God, and the saints, to adore the Cross, and to pray towards the east.
St. John Damascene
Apology Against Those Who Decry Holy Images
Part II