"And as he was praying, the appearance of his countenance was altered, and his raiment became dazzling white. And behold, two men talked with him, Moses and Elijah, who appeared in glory and spoke of his departure, which he was to accomplish at Jerusalem."
Regarding the image:
In this page from a 15th-century Netherlandish 'Biblia Pauperum' (on which see further Image 423) Christ's Transfiguration, flanked by Moses (with horns) and Elisha, is set between its two Old Testament prefigurations: (left) Abraham sees three angels who had accepted his hospitality (Genesis 18:1-3); and (right) King Nebuchadnezzar has three youths placed in a fiery furnace (Daniel 3:25). The page's theme is the mystery of the Trinity: three persons in one divine Nature. As the text top left explains, the three angels signified the Trinity of Persons, but because Abraham worshipped them as one, he indicated the singleness of the Trinity's nature. As the text top right explains, when Nebuchadnezzar saw, to his amazement, a fourth in the surviving burning fiery furnace, like the Son of God, this signified that the three youths represented the Trinity of Persons, and the fourth the singleness of their nature. Both prefigurations point to the Transfiguration's significance in revealing the Trinity. The four prophetic texts all emphasize light and beauty: Psalms 44:3 (45:2); Isaiah 60:1 (top right); Malachi 4:2 (bottom left); Habbakuk 3:4(right); the dazzlement of the glorified body is registered by the figures below Christ. Henry 1987, p.71 Henry, 1987