The veneration of icons in the Fathers

The Orthodox theology of the holy icons: that since the eternal Word became flesh, the image of the invisible God can and must be depicted; that the veneration (proskynesis) given to the icon passes to the prototype, while the worship of latria belongs to God alone; that the icon participates in the reality it represents and is therefore a means of grace. Foundationally established at the Seventh Ecumenical Council (Nicaea II, 787) against the iconoclast emperors, and articulated most fully by St John of Damascus.

Ex 25:1-24
Exodus · Moses · Updated Brenton English Septuagint
¹ And the Lord spoke to Moses, saying, ² Speak to the children of Israel, and take firstfruits of all, who may be disposed in their heart to give; and ye shall take my firstfruits. ³ And this is the offering which ye shall take of them; gold and silver and brass, ⁴ and blue, and purple, and double scarlet, and fine spun linen, and goats’ hair, ⁵ and rams’ skins dyed red, and blue skins, and incorruptible wood, ⁶ and oil for the light, incense for anointing oil, and for the composition of incense, ⁷ and sardius stones, and stones for the carved work of the breastplate, and the full-length robe. ⁸ And thou shalt make me a sanctuary, and I will appear among you. ⁹ And thou shalt make for me according to all things which I show thee in the mountain; even the pattern of the tabernacle, and the pattern of all its furniture: so shalt thou make it. ¹⁰ And thou shalt make the ark of testimony of incorruptible wood; the length of two cubits and a half, and the breadth of a cubit and a half, and the height of a cubit and a half. ¹¹ And thou shalt gild it with pure gold, thou shalt gild it within and without; and thou shalt make for it golden wreaths twisted round about. ¹² And thou shalt cast for it four golden rings, and shalt put them on the four sides; two rings on the one side, and two rings on the other side. ¹³ And thou shalt make staffs [of] incorruptible wood, and shalt gild them with gold. ¹⁴ And thou shalt put the staffs into the rings on the sides of the ark, to bear the ark with them. ¹⁵ The staffs shall remain fixed in the rings of the ark. ¹⁶ And thou shalt put into the ark the testimonies which I shall give thee. ¹⁷ And thou shalt make a propitiatory, a lid of pure gold; the length of two cubits and a half, and the breadth of a cubit and a half. ¹⁸ And thou shalt make two cherubs graven in gold, and thou shalt put them on both sides of the propitiatory. ¹⁹ They shall be made, one cherub on this side, and another cherub on the other side of the propitiatory; and thou shalt make the two cherubs on the two sides. ²⁰ The cherubs shall stretch forth their wings above, overshadowing the propitiatory with their wings; and their faces shall be toward each other, the faces of the cherubs shall be toward the propitiatory. ²¹ And thou shalt set the propitiatory on the ark above, and thou shalt put into the ark the testimonies which I shall give thee. ²² And I will make myself known to thee from thence, and I will speak to thee above the propitiatory between the two cherubs, which are upon the ark of testimony, even in all things which I shall charge thee concerning the children of Israel. ²³ And thou shalt make a golden table of pure gold, in length two cubits, and in breadth a cubit, and in height a cubit and a half. ²⁴ And thou shalt make for it golden wreaths twisted round about, and thou shalt make for it a crown of a handbreadth round about.
The divine command that the Ark of the Covenant be made with two golden cherubim of beaten work facing one another over the Mercy-Seat. The same God who forbids idols in Exodus 20 commands sacred images in Exodus 25. The Fathers and the Seventh Council appeal to this to refute iconoclasm.
Ex 25:18-22; cited at the Seventh Ecumenical Council and by St John of Damascus
Jn 1:14-18
John · Holy Apostle and Evangelist John the Theologian · World English Bible (NT)
¹⁴ The Word became flesh and lived among us. We saw his glory, such glory as of the only born Son of the Father, full of grace and truth. ¹⁵ John testified about him. He cried out, saying, “This was he of whom I said, ‘He who comes after me has surpassed me, for he was before me.’” ¹⁶ From his fullness we all received grace upon grace. ¹⁷ For the law was given through Moses. Grace and truth were realized through Jesus Christ. ¹⁸ No one has seen God at any time. The only born Son, who is in the bosom of the Father, has declared him.
"And the Word became flesh, and lived among us. We saw his glory." St John of Damascus draws the foundational iconographic principle: since the eternal Word has taken visible flesh, He can and must be depicted. To refuse the image is to deny the Incarnation.
Jn 1:14; St John of Damascus, On the Divine Images I (the foundational argument)
Col 1:15-20
Colossians · Holy Apostle Paul · World English Bible (NT)
¹⁵ He is the image of the invisible God, the firstborn of all creation. ¹⁶ For by him all things were created in the heavens and on the earth, visible things and invisible things, whether thrones or dominions or principalities or powers. All things have been created through him and for him. ¹⁷ He is before all things, and in him all things are held together. ¹⁸ He is the head of the body, the assembly, who is the beginning, the firstborn from the dead, that in all things he might have the preeminence. ¹⁹ For all the fullness was pleased to dwell in him, ²⁰ and through him to reconcile all things to himself by him, whether things on the earth or things in the heavens, having made peace through the blood of his cross.
"He is the image (εἰκών) of the invisible God, the firstborn of all creation." Christ Himself is the prototype Icon of the Father. Iconography is grounded in this: the icon participates in the relationship of image and prototype that exists eternally between the Son and the Father.
Col 1:15; St John of Damascus's three Apologies, Argument 1
FideOrth 4.16.1
An Exact Exposition of the Orthodox Faith (De Fide Orthodoxa) · Saint John of Damascus · Ante-Nicene Fathers (Roberts–Donaldson)
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 reverence to each other unless because we are made after God’s image? For as Basil, 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 which the derivative is obtained. Why was it that the Mosaic people honoured on all hands the tabernacle which bore an image and type of heavenly things, or rather of the whole creation? God indeed said to Moses, Look that thou make them after their pattern which was shewed thee in the mount. The Cherubim, too, which o’ershadow the mercy seat, are they not the work of men’s hands? What, further, is the celebrated temple at Jerusalem? Is it not hand-made and fashioned by the skill of men?
St John of Damascus: "But since some find fault with us for worshipping and honoring 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... I do not worship matter, I worship the Creator of matter who became matter for my sake, and willed to take His abode in matter; who worked out my salvation through matter." The decisive Eastern argument: matter is honoured because the Word has hallowed matter by becoming flesh.
St John of Damascus, Exact Exposition IV.16
FideOrth 4.16.3
An Exact Exposition of the Orthodox Faith (De Fide Orthodoxa) · Saint John of Damascus · Ante-Nicene Fathers (Roberts–Donaldson)
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 common. 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 valour 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.
St John of Damascus distinguishes the worship (latreia) due to God alone from the veneration (proskynesis) of icons that passes to the prototype: "The honour paid to the image passes over to the prototype." The phrase, drawn from St Basil's On the Holy Spirit, becomes the dogmatic touchstone at Nicaea II.
St John of Damascus, Exact Exposition IV.16; quoting St Basil, On the Holy Spirit XVIII.45
Nic2 11.1.3
The Seventh Ecumenical Council (Nicaea II, 787) · The Seven Holy Ecumenical Councils · Ante-Nicene Fathers (Roberts–Donaldson)
To our most religious and most serene princes, Constantine and Irene his mother. Tarasius, the unworthy bishop of your God-protected royal city, new Rome, and all the holy Council which met at the good pleasure of God and upon the command of your Christ-loving majesty in the renowned metropolis of Nice, the second council to assemble in this city.
From the Definition of the Seventh Ecumenical Council (Nicaea II, 787): "We define that the holy icons, whether in colour, mosaic, or any other material, are suitably exhibited in the holy churches of God, on the sacred vessels and vestments, on the walls, furniture, in houses, and on the highways; the icons of our Lord and God and Saviour Jesus Christ, of the spotless Lady Mother of God, of the holy angels, and of all holy people. The more frequently they are seen in artistic representation, the more readily are men lifted up to the memory of their prototypes and to a longing after them."
Seventh Ecumenical Council (Nicaea II), Definition of Faith
Nic2 11.1.4
The Seventh Ecumenical Council (Nicaea II, 787) · The Seven Holy Ecumenical Councils · Ante-Nicene Fathers (Roberts–Donaldson)
Christ our God (who is the head of the Church) was glorified, most noble princes, when your heart, which he holds in his hands, gave forth that good word bidding us to assemble in his name, in order that we might strengthen our hold on the sure, immovable, and God-given truth contained in the Church’s dogmas. As your heads were crowned with gold and most brilliant stones, so likewise were your minds adorned with the precepts of the Gospel and the teachings of the Fathers. And being the disciples and companions, as it were, of those whose sounds went forth into all the earth, ye became the leaders in the way of piety of all who bore the name of Christ, setting forth clearly the word of truth, and giving a brilliant example of orthodoxy and piety; so that ye were to the faithful as so many burning lamps. The Church which was ready to fall, ye upheld with your hands, strengthening it with sound doctrine, and bringing into the unity of a right judgment those who were at variance. We may therefore well say with boldness that it was through you that the good pleasure of God brought about the triumph of godliness, and filled our mouth with joy and our tongue with gladness. And these things our lips utter with a formal decree. For what is more glorious than to maintain the Church’s interests; and what else is more calculated to provoke our gladness?
The Council distinguishes worship from veneration explicitly: "To these should be given salutation and honourable reverence (ἀσπασμός καὶ τιμητικὴ προσκύνησις), not indeed that true worship of faith (λατρεία) which pertains alone to the divine nature; but to these, as to the figure of the precious and life-giving Cross and to the holy Gospels and to the other sacred objects, incense and lights may be offered according to ancient pious custom." The dogmatic distinction Western polemics often miss.
Seventh Ecumenical Council (Nicaea II), Definition of Faith
Trull 1.82.1
The Quinisext Council in Trullo (692) · The Seven Holy Ecumenical Councils · Ante-Nicene Fathers (Roberts–Donaldson)
Canon LXXXI.
Canon 82 of the Quinisext Council in Trullo (692), pre-iconoclastic, ordered that Christ be depicted henceforth in His human form rather than under the symbol of the Lamb of John 1:29: "so that all may understand by means of it the depths of the humiliation of the Word of God, and that we may recall to our memory His conversation in the flesh." The Council enforces the iconographic consequence of the Incarnation.
Quinisext Council, Canon 82