The intention of those who venerate is what’s important

So with regard to images we must manifest the truth, and take into account the intention of those who make them. If it be in very deed for the glory of God and of His saints to promote goodness, to avoid evil, and save souls, we should receive and honour and worship them as images, and remembrances, likenesses, and the books of the illiterate. We should love and embrace them with hand and heart as reminders of the incarnate God, or His Mother, or of the saints, the participators in the sufferings and the glory of Christ, the conquerors and overthrowers of Satan, and diabolical fraud. If any one should dare to make an image of Almighty God, who is pure Spirit, invisible, uncircumscribed, we reject it as a falsehood. If any one make images for the honour and worship of the Devil and his angels, we abhor them and deliver them to the flames. Or if any one give divine honours to the statues of men, or birds, or reptiles, or any other created thing, we anathematise him.

St. John Damascene
Apology Against Those Who Decry Holy Images
Part II

We honor God by venerating his holy ones

Whenever, then, you see Christians adoring the Cross, know that they are adoring the Crucified Christ, not the mere wood.* If, indeed, they honoured wood as wood, they would be bound to worship trees of whatever kind, as you, O Israel, worshipped them of old, saying to the tree and to the stone, “Thou art my God and didst bring me forth.” (Jer. 2.27) We do not speak either to the Cross or to the representations of the saints in this way. They are not our gods, but books which lie open and are venerated in churches in order to remind us of God and to lead us to worship Him. He who honours the martyr honours God, to whom the martyr bore testimony. He who worships the apostle of Christ worships Him who sent the apostle. He who falls at the feet of Christ’s mother most certainly shows honour to her Son.

St. John Damascene
Apology Against Those Who Decry Holy Images
Quoted from Leo of Neapolis, Against the Jews

We worship Christ through venerating the Cross

If you, O Jew, reproach me saying that I adore the wood of the Cross as God, why do you not reproach Jacob, who worshipped on the point of his staff? Now it is evident that he was not worshipping wood. So with us; we are worshipping Christ through the Cross, not the wood of the Cross.

St. John Damascene
Apology Against Those Who Decry Holy Images
Quoted from Leo of Neapolis, Against the  Jews

Our honor of the icon is honor to the original

The image of the King is also called the king, and there are not two kings in consequence. Neither is power divided, nor is glory distributed. just as the reigning power over us is one, so is our homage one, not many, and the honor given to the image reaches back to the original. What the image is in the one case as a representation, that the Son is by His humanity, and as in art likeness is according to form, so in the divine and incommensurable nature union is effected in the indwelling Godhead.

St. John Damascene
Apology Against Those Who Decry Holy Images
Quoted from Amphilochios, On the Holy Spirit

Epiphanius is falsely attributed to rejecting icons

If, again, you object that the great Epiphanius thoroughly rejected images, I would say in the first place the work in question is fictitious and unauthentic. It bears the name of some one who did not write it, which used to be commonly done. Secondly, we know that blessed Athanasius objected to the bodies of saints being put into chests, and that he preferred their burial in the ground, wishing to set at nought the strange custom of the Egyptians, who did not bury their dead under ground, but set them upon beds and couches. Thus, supposing that he really wrote this work, the great Epiphanius, wishing to correct something of the same kind, ordered that images should not be used. The proof that he did not object to images, is to be found in his own church, which is adorned with images to this day. Thirdly, the exception is not a law to the Church, neither does one swallow make summer, as it seems to Gregory the theologian, and to the truth. Neither can one expression overturn the tradition of the whole Church which is spread throughout the world.

St. John Damascene
Apology Against Those Who Decry Holy Images
Part I

Unwritten traditions are handed down in the Church

How do we know the Holy place of Calvary, or the Holy Sepulchre? Does it not rest on a tradition handed down from father to son? It is written that our Lord was crucified on Calvary, and buried in a tomb, which Joseph hewed out of the rock; (Mt. 27.60) but it is unwritten tradition which identifies these spots, and does more things of the same kind. Whence come the three immersions at baptism, praying with face turned towards the east, and the tradition of the mysteries?* Hence St Paul says, “Therefore, brethren, stand fast, and hold the traditions which you have learned either by word, or by our epistle.” (II Thess. 2.15) As, then, so much has been handed down in the Church, and is observed down to the present day, why disparage images?

St. John Damascene
Apology Against Those Who Decry Holy Images
Part I

The relics of the Apostles were miraculous so shouldn’t the Saints be glorified?

…the image of Him who is visible to flesh is burnt into my soul. The shadow and winding sheet and relics of the apostles cured sickness, and put demons to flight. (Acts 5.15) How, then, shall not the shadow and the statues of the saints be glorified?

St. John Damascene
Apology Against Those Who Decry Holy Images
Part I

Christ has deified our flesh

From the time that God the Word became flesh He is as we are in everything except sin, and of our nature, without confusion. He has deified our flesh for ever, and we are in very deed sanctified through His Godhead and the union of His flesh with it.

St. John Damascene
Apology Against Those Who Decry Holy Images
Part I

The images of the Saints are honored in the Church

We depict Christ as our King and Lord, and do not deprive Him of His army. The saints constitute the Lord’s army. Let the earthly king dismiss his army before he gives up his King and Lord. Let him put off the purple before he takes honor away from his most valiant men who have conquered their passions. For if the saints are heirs of God, and co-heirs of Christ, (Rom. 8.17) they will be also partakers of the divine glory of sovereignty. If the friends of God have had a part in the sufferings of Christ, how shall they not receive a share of His glory even on earth? “I call you not servants,” our Lord says, “you are my friends.” (Jn. 15.15) Should we then deprive them of the honor given to them by the Church?

St. John Damascene
Apology Against Those Who Decry Holy Images
Part I