When, then, you see a Christian worshipping the Cross, know that his adoration is not given to the wood, but to Christ Crucified. We might as well worship all wood, as Israel worshipped woods and trees, saying, ‘Thou art my God, and Thou hast brought me forth.’ It is not so with us. We keep in churches and in our houses a remembrance and a representation of our Lord’s sufferings and of those who fought for Him, doing everything for our Lord’s sake.
Once more. Tell me, O Jew, what law authorised Moses to worship Jethor, his brother-in-law, and an idolator? Or Jacob to worship Pharao, and Abraham the sons of Emmor? They were just men and prophets. Again, Daniel worshipped the impious Nabuchodonosor. For if they so acted on account of life in this world, why do you reproach me for worshipping the holy memories of the saints, whether in books or pictures, their combats and sufferings, which are a daily source of good to me, and will help me to lasting and eternal life?
Translated by Mary H. Allies. Edition opa.patristics.john-of-damascus.on-holy-images.en.