From Greek orthodoxa, "correct opinion"; term used by Irenaeus (second century) to describe the beliefs that Jesus originated and the apostles proclaimed. In Christian theology, "orthodoxy" describes the acceptance of certain essential beliefs about the person and work of Jesus Christ. When capitalized, "Orthodoxy" may refer to churches-such as the Eastern Orthodox and Russian Orthodox-whose origins trace to the eastern half of the ancient Roman empire. See apostle; heresy; Irenaeus of Lyons.