Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
(order), as applied to the clergy, has been the occasion of controversy. Many contend that it is adopted from the Roman language, and used by Tertullian and others of the classic sense, to exhibit the patrician rank of the clergy, like the ordo senatorius of the Romans. The fact is, that the word is used to denote the difference between the clergy and the people- the ordo ecclesiasticus and the laity; and in this sense it has been understood since the close of the second century. See Riddle, Christiati Antiquities, p. 203, 212.