Answer
The First Council of Nicea (or Nicaea) convened in AD 325 and issued statements on the Trinity and the divinity of Christ. The Second Council of Nicea (AD 787) was called to definitively settle the issue of the use of images in worship. The use and eventual worship of images had become an issue in the church. Many who worshiped or venerated images claimed it was not the images themselves but the personages that the images represented—Mary, the angels, various saints, etc.—that were being venerated.
For about 300 years, some church leaders had been attempting to stop the veneration of images, while others encouraged it. The Roman Emperors Leo III and his son Constantine V had tried to stop the practice, but they were opposed by church leaders who sanctioned the practice. The Second Council of Nicea was called to decide the issue.
Those who opposed the use of images did so on the grounds that it violated the Second Commandment: “You shall not make for yourself a carved image, or any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth. You shall not bow down to them or serve them, for I the Lord your God am a jealous God” (Exodus 20:4–5, ESV). Those who encouraged the use of images in worship said that the practice was similar to the worship of the Angel of the Lord in the Old Testament. In using this argument, the advocates assumed that the Angel of the Lord is not a theophany, an actual appearance of God, but simply a messenger that represents God and accepts worship on His behalf.
Ultimately, the Second Council of Nicea decided that the use of images was appropriate as long as the worship given to the images and those whom they represent was different from the worship of God. The council made a fine distinction between the worship of images and the worship of God, giving each type of worship a term of its own: proskynesis (“bowing before and venerating”) was the worship of images, and latria was the adoration and worship reserved for God alone.
In doing so, the Second Council of Nicea developed a category of lesser worship that is not found in Scripture. Even if the Angel of the Lord is not an appearance of God Himself but rather a messenger who accepts worship on behalf of God, that is quite a different thing from people crafting images and then worshiping them or, through them, worshiping other created beings such as saints and angels. Today, the Catholic Church teaches three levels of honor or worship: dulia, hyperdulia, and latria.
The Protestant Reformers thoroughly rejected such extrabiblical distinctions in worship and so rejected the decision of the Second Council of Nicea. Churches of the Reformation eliminated the use of any images in the worship of God and also forbade prayers and any kind of worship directed to Mary, saints, or angels—with or without images.
