Since, therefore…we have explored the depths of divine knowledge, we are obliged to carry out in fullest detail what the Master has commanded us to do at stated times. He has ordered the sacrifices to be offered and the services to be held, and this not in a random and irregular fashion, but at definite times and seasons. He has, moreover, Himself, by His sovereign will determined where and by whom He wants them to be carried out. Thus all things are done religiously, acceptable to His good pleasure, dependent on His will. Those, therefore, that make their offerings at the prescribed times are acceptable and blessed; for, since they comply with the ordinances of the Master, they do not sin. Special functions are assigned to the high priest; a special office is imposed upon the priests; and special ministrations fall to the Levites. The layman is bound by the rules laid down for the laity. — from Epistle to the Corinthians, 40.
St. Clement I (circa 96 A.D.) is the fourth pope and the first apostolic father of the Catholic Church. While little is known about his life, his Epistle to the Corinthians is the earliest piece of Christian writing outside the New Testament and has been a touchstone for Christian sects because of the lucidity and simplicity with which it communicates the faith. It is also the earliest evidence that the Church hierarchy of laity (a term coined by Clement) — deacons, priests, and bishops — was already well established by the time he was writing.