The Second Council of Nicaea is regarded by Catholics and Eastern Orthodox Christians as the 7th ecumenical council, meaning that they affirm it to be infallible in matters of doctrine. It was a response to the Iconoclast controversy, which surrounded the permissibility of creating and venerating religious images.
It overturned the earlier Council of Hieria (754 AD), which had also declared itself the 7th ecumenical council, and forbade the use of icons and images in worship:
Council of Hieria (754 AD)
Supported by the Holy Scriptures and the Fathers, we declare unanimously, in the name of the Holy Trinity, that there shall be rejected and removed and cursed out of the Christian Church every likeness which is made out of any material and colour whatever by the evil art of painters.
Whoever in future dares to make such a thing, or to venerate it, or set it up in a church, or in a private house, or possesses it in secret, shall, if bishop, presbyter, or deacon, be deposed; if monk or layman, be anathematised, and become liable to be tried by the secular laws as an adversary of God and an enemy of the doctrines handed down by the Fathers.
8 If anyone ventures to represent the divine image (χαρακτήρ) of the Word after the Incarnation with material colours, let him be anathema!
9 If anyone ventures to represent in human figures, by means of material colours, by reason of the incarnation, the substance or person (ousia or hypostasis) of the Word, which cannot be depicted, and does not rather confess that even after the Incarnation he [i.e., the Word] cannot be depicted, let him be anathema!
The above council was summoned by the Iconoclast Byzantine emperor Constantine V (718-775 AD). However, this council would only stand for a few decades, as after Constantine V died, the empress Irene of Athens (750-806 AD), who was sympathetic to icons, convoked the Second Council of Nicaea. This same empress Irene would later gouge out the eyes of her own son, Constantine VI, and then imprison him until he died, in order to become sole ruler of the Byzantine empire.
This council made image veneration into a sine qua non of Salvation. It did not merely decree that image veneration was permissible, but rather was an essential part of orthodoxy, without which someone was to be cut off from the Body of Christ:
Second Council of Nicaea (787 AD)
If anyone does not salute such representations as standing for the Lord and his saints, let him be anathema.
Let them who do not venerate the holy and venerable images be anathema!
To those who have a doubtful mind and do not confess with their whole hearts that they venerate the sacred images, anathema!
We salute the venerable images. We place under anathema those who do not do this.
[We have likewise decreed] that these images are to be reverenced (προσκυνεῖν), that is, salutations are to be offered to them. The reason for using the word is, that it has a two-fold signification. For κυνεῖν in the old Greek tongue signifies both "to salute" and "to kiss". And the preposition προς gives to it the additional idea of strong desire towards the subject; as for example, we have φέρω and προσφέρω, κυρῶ and προσκυρῶ, and so also we have κυνέω and προσκυνέω. Which last word implies salutation and strong love; for that which one loves he also reverences (προσκυνεῖ) and what he reverences that he greatly loves, as the everyday custom, which we observe towards those we love, bears witness, and in which both ideas are practically illustrated when two friends meet together.
If anyone does not accept this our Holy and Ecumenical Seventh Synod, let him be anathema from the Father and the Son and the Holy Ghost, and from the seven holy Ecumenical Synods!
Now anathema is nothing less than complete separation from God. For if any are quarrelsome and will not obediently accept what has now been decreed, they but kick against the pricks, and injure their own souls in their fighting against Christ.
Notice the following from the above:
Any person who has even the slightest respect for the purity and content of the Gospel of grace is appalled at what they have just read. This council has turned the Gospel into veneration of pieces of wood and stone, which have been painted to look like Biblical figures.
Often, the discussion around images will get bogged down in the distinction, or lack thereof, between worship and veneration. Such a discussion is just a distraction. The real issue is that they have made the veneration of images a requirement for Salvation, which is a flagrant, blasphemous assault on the pure Gospel, of the worst sort.
The Gospel message is clearly elucidated in Scripture (1 Corinthians 15:1-8), it is defined as God's message of Salvation (Romans 1:16: 1 Corinthians 1:17-18, Ephesians 1:12-14: 2 Timothy 1:10), it is carefully and bitterly guarded against any attempt to add anything at all to it (Acts 15:5-11: 2 Corinthians 11:3-4, Galatians 1:6-9, 2:11-14), and it does not include venerating icons. The Second Council of Nicaea violently overthrew the Bible's testimony of the Gospel, and replaced it with another Gospel, which included something expressly forbidden in both Testaments.
Beginning with the Biblical case against icon veneration, it should be noted that three times in the New Testament, veneration is rejected by a saint (Peter, Acts 10:25-26), as well as an angel (Revelation 19:10, 22:8-9) - both of which are subjects of Catholic and Orthodox iconography.
Firstly, notice Peter's response to Cornelius's veneration in Acts chapter 10:
Acts 10:25-26
25 And as Peter was coming in, Cornelius met him, and fell down at his feet, and worshipped him.
26 But Peter took him up, saying, Stand up; I myself also am a man.
There is no indication that Cornelius, "A devout man, and one that feared God with all his house" (Acts 10:2), either thought that Peter was God, or wanted to worship him as God. Instead, he was venerating Peter, and Peter denies it, saying "I myself also am a man". In saying this, Peter is teaching that men should not venerate other men, but rather God alone should be venerated in such a manner. If veneration of the actual human being Peter was denied, how much more should the veneration of a piece of wood or metal painted to look like Peter be denied.
Secondly, twice in the vision that the Apostle John receives in the book of Revelation, he errantly worships an angel:
Revelation 19:10
10 And I fell at his feet to worship him. And he said unto me, See thou do it not: I am thy fellowservant, and of thy brethren that have the testimony of Jesus: worship God: for the testimony of Jesus is the spirit of prophecy.
Revelation 22:8-9
8 And I John saw these things, and heard them. And when I had heard and seen, I fell down to worship before the feet of the angel which shewed me these things.
9 Then saith he unto me, See thou do it not: for I am thy fellowservant, and of thy brethren the prophets, and of them which keep the sayings of this book: worship God.
As in the case of Cornelius, there is no indication that the Apostle John thought that this angel was God, or was worshipping the angel as God. Instead, he honors the angel by bowing before it, and twice, the angel denies the veneration, saying "I am thy fellowservant". The angel also states, as part of his denial, that he is "of thy brethren the prophets", which would seem to extend this denial of veneration to prophets more generally. He concludes his denial of John's veneration with an instruction to "worship God".
In addition to these New Testament examples, the Bible is filled with verses which, in context, prohibit the kind of image veneration done by Roman Catholics and Eastern Orthodox:
Also, special consideration must be given to the case of venerating images of Christ, specifically. If honor given to images pass to their prototypes, then why are we not to worship an image of Christ as if it were God itself? Does the veneration given to an image of Christ become amplified into true and proper worship somehow? Where is such a mechanism described in Scripture?
The argument from Church history against image veneration is overwhelming. It is impossible to view what exists of early Christian writings, and maintain that the Byzantine iconodulia that was mandated by the Second Council of Nicaea was a universal and apostolic practice.
Beginning historical citations against icon veneration is Origen (185-253 AD), a prolific Christian writer based in Alexandria. While Origen is widely known for his opposition to the use of images, the following quotation is particularly interesting because it is drawn from The Philocalia of Origen - a collection of his writings compiled by two influential early Christian theologians, Basil of Caesarea (330-379 AD), and Gregory of Nazianzus (329-390 AD):
Origen (185-253 AD) - The Philocalia of Origen, Chapter 19
3 For though the perverted doctrine, backed up with much instruction, has been able to implant in the minds of the many the belief that images are gods, and that things made of gold, and silver, and ivory, and stone, are worthy of worship; common sense, nevertheless, forbids us to think that God is by any means corruptible matter, or that He is honoured when He is fashioned by men in forms of dead matter, supposed to pictorially or symbolically represent Him.
The above is a witness to three early, prominent, well-informed Christians holding the opinion that God is not honored by people fashioning images "supposed to pictorially or symbolically represent Him". Also, in context, the statement is never qualified in such a way that would suggest that Christians, while not doing whatever Origen is condemning here, nevertheless have their own proper images of Christ. That is absent. Instead, a statement like this seeks to distinguish Pagans, who use images, and Christians, who do not.
Next, Marcus Minucius Felix (250 AD) was an early Latin Christian apologist who lived in Rome. His sole surviving work, Octavius, is a dialogue between a Pagan named Caecilius and a Christian named Octavius. In it, Caecilius asks why Christians have "no acknowledged images", and Octavius responds:
Minucius Felix (250 AD) - Octavius
10 (Caecilius) I purposely pass over many things, for those that I have mentioned are already too many; and that all these, or the greater part of them, are true, the obscurity of their vile religion declares. For why do they endeavour with such pains to conceal and to cloak whatever they worship, since honourable things always rejoice in publicity, while crimes are kept secret? Why have they no altars, no temples, no acknowledged images? Why do they never speak openly, never congregate freely, unless for the reason that what they adore and conceal is either worthy of punishment, or something to be ashamed of?
32 (Octavius) But do you think that we conceal what we worship, if we have not temples and altars? And yet what image of God shall I make, since, if you think rightly, man himself is the image of God? What temple shall I build to Him, when this whole world fashioned by His work cannot receive Him?
Above, in his dialogue, Felix has the Pagan Caecilius use the Christians' lack of images as a critique against Christianity. He then has Octavius respond - "What image of God shall I make, since, if you think rightly, man himself is the image of God?". It would not make sense to have a Pagan critique Christians for not having images if, at that time, Christians actually used and venerated images.
Next, Lactantius (250-325 AD) was an early Christian writer, and advisor to Emperor Constantine (272-337 AD). As part of a multi-volume work on Christian theology, he writes:
Lactantius (250-325 AD) - Divine Institutes, Book 2
19 Wherefore it is undoubted that there is no religion wherever there is an image. For if religion consists of divine things, and there is nothing divine except in heavenly things; it follows that images are without religion, because there can be nothing heavenly in that which is made from the earth.
The above statement - "there is no religion wherever there is an image" - is not qualified in the work in terms of proper image veneration. If veneration of images were a universal, early Christian practice, we would expect the above statement to be expounded upon as being in regard to the improper use of images, and a proper use of them to be taught, or referenced. Rather, the section on images in this work contains no such qualification, because it was intended to be unqualified - Christianity is where images are not.
Next, one of the earliest Church historians, Eusebius (265-339 AD), after being asked for an image of Christ by the sister of Emperor Constantine, responded with a letter saying:
Eusebius (265-339 AD) - Letter to Constantia
But if you mean to ask of me the image, not of His form transformed into that of God, but that of the mortal flesh before its transformation, can it be that you have forgotten that passage in which God lays down the law that no likeness should be made either of what is in heaven or what is in the earth beneath? Have you ever heard anything of the kind either yourself in church or from another person? Are not such things banished and excluded from churches all over the world, and is it not common knowledge that such practices are not permitted to us alone?
Once - I do not know how - a woman brought me in her hands a picture of two men in the guise of philosophers and let fall the statement that they were Paul and the Saviour - I have no means of saying where she had had this from or learned such a thing. With the view that neither she nor others might be given offence, I took it away from her and kept it in my house, as I thought it improper that such things ever be exhibited to others, lest we appear, like idol worshippers, to carry our God around in an image. I note that Paul instructs all of us not to cling any more to things of the flesh; for, he says, though we have known Christ after the flesh, yet now henceforth know we Him no more.
Above, the Church historian Eusebius says that, in essence, the use of images was unknown to churches of his day - "Are not such things banished and excluded from churches all over the world, and is it not common knowledge that such practices are not permitted to us alone?".
Next, the pre-Nicene Council of Elvira (306 AD) had a canon which prohibited images in the church:
Council of Elvira (306 AD) - Canon 36
36 It was decided that there should not be pictures in the church, so that which is worshiped and adored should not be painted on the walls.
The above is a rather straightforward, unqualified banning of imagery in church of "that which is worshiped and adored", which is to say, Christ. Qualifications can be read into the text, but that is true of any text. The authors did not feel the need to qualify their statement - a statement which would be silly if image veneration of Jesus Christ were an accepted part of Christian worship at the time.
Next, there is a very strong passage on the topic from Epiphanius of Salamis (310-403 AD), wherein he relays an interesting story of him encountering an image hanging in a church that he visited:
Epiphanius (310-403 AD) - Letter to John, Bishop of Jerusalem
9 Moreover, I have heard that certain persons have this grievance against me: When I accompanied you to the holy place called Bethel, there to join you in celebrating the Collect, after the use of the Church, I came to a villa called Anablatha and, as I was passing, saw a lamp burning there. Asking what place it was, and learning it to be a church, I went in to pray, and found there a curtain hanging on the doors of the said church, dyed and embroidered. It bore an image either of Christ or of one of the saints; I do not rightly remember whose the image was. Seeing this, and being loth that an image of a man should be hung up in Christ's church contrary to the teaching of the Scriptures, I tore it asunder and advised the custodians of the place to use it as a winding sheet for some poor person. They, however, murmured, and said that if I made up my mind to tear it, it was only fair that I should give them another curtain in its place. As soon as I heard this, I promised that I would give one, and said that I would send it at once.
Since then there has been some little delay, due to the fact that I have been seeking a curtain of the best quality to give to them instead of the former one, and thought it right to send to Cyprus for one. I have now sent the best that I could find, and I beg that you will order the presbyter of the place to take the curtain which I have sent from the hands of the Reader, and that you will afterwards give directions that curtains of the other sort - opposed as they are to our religion - shall not be hung up in any church of Christ. A man of your uprightness should be careful to remove an occasion of offense unworthy alike of the Church of Christ and of those Christians who are committed to your charge.
Epiphanius tears the image, and states that hanging images in churches is "contrary to the teaching of the Scriptures". This a forceful stance against the mere presence of images in churches, similar to the Council of Elvira. As in that case, this behavior and attitude would not make sense, if early Christians had a practice of venerating images.
For more than 20 additional quotations, see this article by James Attebury.
In light of the above evidence, it seems clear that the early Christians were not venerating icons of saints, angels, and the Lord, in the manner mandated under pain of damnation to Hell by the Second Council of Nicaea.
In conclusion, Jesus did not die on the cross, and bear our sins, and rise from the dead, so that we would have to venerate icons to be saved. The Second Council of Nicaea, accepted by most of the denominations who hold to the "baton passing" view of Apostolic succession, is an irreparable corruption of the Gospel, and what it mandates has no place in the Bible, or in the early Church.
Any Christian who wishes to please the Lord Jesus Christ, and protect the integrity of the Gospel, must reject the Second Council of Nicaea, and all of its unbiblical accretions.