Wooden panel with a painting, usually in tempera, of a holy person or one of the traditional images of Orthodox Christianity, the religion of the Byzantine empire practised today mainly in Greece and Russia. The word also has a range of related but disparate meanings, from the abstract and philosophical to the purely literal. For example, it is still used in modern Greek to mean an image or picture in the ordinary sense. In antiquity, Platonists and Neo-Platonists held that the material, earthly world reflects, or is the image of, the higher and divine cosmos; the Old Testament provides the theme of man as the icon of God in the temple of the world; and St Paul declared that ‘Christ is the icon of God’ (2 Corinthians 4:4). Thus the idea of the icon is associated with cosmology and the theology of the Incarnation. In the Early Christian period, disputes over such questions as whether or not God can be known or depicted or the invisible can be seen were part of an intense debate surrounding the acceptability, meaning and function of images of Christ. . . . The theory and belief system of icons was developed by theologians between the 4th and the 9th centuries, though only a few icons survive from then and up to the 12th century. Once established, however, the doctrinal principles never changed, and the study of icons is as much a matter of theology as of art. Subject-matter, form and composition did not deviate from the established dogma on which they depended; indeed, icons have been called theology in colour (Trubetskoy).