Trinity: A grouping or relationship of three divine persons thought in some way to be equivalent or identical to each other--as is the case in the Christian trinity (Father, Son, and Holy Ghost) or the Egyptian solar trinity (Horus, Ra, and Atun--the sun-gods associated with the morning, noonday, and setting sun). The first Christian missionaries to Ireland were greatly aided by the fact that Irish mythology already contained an idea of trinity in the form of three-headed or three-personed gods, as macculloch notes in The Religion of the Ancient Celts (34, referenced. In Hopper 203). Contrast with a triad, a group of three loosely connected with each other in mythology, philosophy, or poetry. In patristic and medieval literature, a number of theological treatises survive pertaining to the trinity--the most influential probably being Saint Augustine's De Trinitate. Many heretical groups originated in disputes concerning the nature of the trinity (see heresy for more information). The concept of trinity strongly influences Dante's Divine Comedy. To mimic the nature of a threefold deity, Dante writes his poem in terza rima (with sets of three interlocking rhymes), he divides the work into three sections (Inferno, Purgatorio, and Paradiso), finally, he subdivides each section into 33 cantos. Even Satan himself in the work appears as a three-headed, six-winged monster that mimics the tripartite structure of the Godhead. Such numerology is typical of many medieval writings.