Apparently from the earliest days of the Church, there has been a fairly constant trend towards decoration of worship space. In most cases, this has been a matter of placing images of people, places or events from Church history (or the Bible) in prominent view. There may have been several reasons for this, but in any case the practice greatly accelerated during and after the reign of the Emperor Justinian, who died in 565 A.D.
What were the images for? First of all, they certainly served as an educational tool. In an age when illiteracy approached 100% in many congregations, the only way to communicate may have been with pictures. Even today, Greek Orthodox icons, formalized, richly decorated pictures of saints, the Virgin Mary, etc., are called “windows into heaven”. In the West, the images early became more realistic, whereas in the East, they became more stylized. Another reason for having them may well have been that there were always statues and paintings of “gods” in pagan temples, and early Christians may well have wanted to have places of worship that, well, looked like places of worship. Whatever the reason, most of the earliest places of worship that have come to light have provisions for images within them.
That’s not to say that everybody agreed that it was a good idea. Some did not. Some early Christians, and indeed, some Christians at every period have had a nagging feeling that all those paintings and statues are an awful lot like the idols in pagan temples, and many also have also been uncomfortable with the veneration of saints and others as being too much like worship to be comfortable. The dramatic rise of Islam, originally a Christian heresy that ended up being an outright competitor, fed this discomfort, for in Mosques (and other places, for that matter), any sort of realistic image, or any kind of an image of any living thing is strictly forbidden. The first thing Moslem conquerors did when they took over a Christian community was to paint over all the images. This actually made them some converts in Greece and Asia Minor, where the Greek Orthodox Church was deeply into very expensive icons.
In the second quarter of the 8th Century, encouraged by their Moslem neighbors, Greek Christians in Asia Minor broke out into a half-Century-long period of violence aimed at destroying religious imagery. Historians call this period “the iconoclasm”, which means “the breaking of the images”. During this time, and off and on for fifty years or more, both the government and individuals ran rampant over the countryside, destroying religious images. This was strongly resisted in Rome and the West, and this difference over iconography was perhaps as important in the coming Great Schism as the western addition of “the filioque” to the Nicene Creed.