Philip Schaff discusses the transition of the idea of a special priesthood, sacrifice, and altar from Jewish and heathen traditions into the Christian church, leading to the development of a distinct clergy class. He highlights the shift from the apostolic age where preaching and teaching were open to all believers to the emergence of a separate clergy class in the second century, eventually leading to the exaltation and separation of clergy from secular affairs. The term 'clergy' evolved to exclusively refer to Christian ministers, especially bishops, emphasizing their unique role as mediators between God and the laity, with celibacy becoming optional but secular engagements being forbidden after the third century.

The idea and institution of a special priesthood, distinct from the body of the people, with the accompanying notion of sacrifice and altar, passed imperceptibly from Jewish and heathen reminiscences and analogies into the Christian church. The majority of Jewish converts adhered tenaciously to the Mosaic institutions and rites, and a considerable part never fully attained to the height of spiritual freedom proclaimed by Paul, or soon fell away from it. He opposed legalistic and ceremonial tendencies in Galatia and Corinth; and although sacerdotalism does not appear among the errors of his Judaizing opponents, the Levitical priesthood, with its three ranks of high priest, priest, and Levite, naturally furnished an analogy for the threefold ministry of bishop, priest, and deacon, and came to be regarded as typical of it. Still less could the Gentile Christians, as a body, at once emancipate themselves from their traditional notions of priesthood, altar, and sacrifice, on which their former religion was based. Whether we regard the change as an apostasy from a higher position attained, or as a reaction of old ideas never fully abandoned, the change is undeniable, and can be traced to the second century. The church could not long occupy the ideal height of the apostolic age, and as the pentecostal illumination passed away with the death of the apostles, the old reminiscences began to reassert themselves1. In the apostolic church preaching and teaching were not confined to a particular class, but every convert could proclaim the gospel to unbelievers, and every Christian who had the gift could pray and teach and exhort in the congregation2. The New Testament knows no spiritual aristocracy or nobility, but calls all believers "saints", though many fell far short of their vocation. Nor does it recognize a special priesthood in distinction from the people, as mediating between God and the laity. It knows only one high priest, Jesus Christ, and clearly teaches the universal priesthood, as well as universal kingship, of believers3. It does this in a far deeper and larger sense than the Old (see Exodus 19:6); in a sense, too, which even to this day is not yet fully realized. The entire body of Christians are called "clergy" (klhrwn), a peculiar people, the heritage of God4. After the gradual abatement of the extraordinary spiritual elevation of the apostolic age, ... the distinction of a regular class of teachers from the laity became more fixed and prominent. This appears first in Ignatius, who, in his high episcopalian spirit, considers the clergy the necessary medium of access for the people to God. "Whoever is within the sanctuary (or altar), is pure; but he who is outside of the sanctuary is not pure; that is, he who does anything without bishop and presbytery and deacon, is not pure in conscience". ... During the third century it became customary to apply the term "priest" directly and exclusively to the Christian ministers, especially the bishops. In the same manner the whole ministry, and it alone, was called "clergy", with a double reference to its presidency and its peculiar relation to God. It was distinguished by this name from the Christian people or "laity". Thus the term "clergy", which first signified the lot by which office was assigned (Acts 1:17, 25), then the office itself, then the persons holding that office, was transferred from the Christians generally to the ministers exclusively. ... With the exaltation of the clergy appeared the tendency to separate them from secular business, and even from social relations – from marriage, for example – and to represent them, even outwardly, as a caste independent of the people, and devoted exclusively to the service of the sanctuary. They drew their support from the church treasury, which was supplied by voluntary contributions and weekly collections on the Lord's Day. After the third century they were forbidden to engage in any secular business, or even to accept any trusteeship. Celibacy was not yet in this period enforced, but left optional. From History of the Christian Church, chapter 42. 1 Joseph Renan, looking at the gradual development of the hierarchy out of the primitive democracy, from his secular point of view, calls it "the most profound transformation" in history, and a triple abdication: first the club (the congregation) committing its power to the bureau or the committee (the college of presbyters), then the bureau to its president (the bishop) who could say: "Je suis le club" [I am the club], and finally the presidents to the pope as the universal and infallible bishop; the last process being completed in the Vatican Council of 1870. See his L'Eglise chretienne, 1879, p. 88, and his English Conferences (Hibbert Lectures, 1880), p. 90. 2 Compare Acts 8:4; 9:27; 13:15; 18:26,28; Romans 12:6; 1 Corinthians 12:10,28; 14:1-6,31. Even in the Jewish Synagogue the liberty of teaching was enjoyed, and the elder could ask any member of repute, even a stranger, to deliver a discourse on the Scripture lesson (Luke 4:17; Acts 17:2). 3 1 Peter 2:5,9; Revelation 1:6; 5:10; 20:6). 4 1 Peter 5:3. Here Peter warns his fellow-presbyters not to lord it over the klhrwn, that is, the lot or inheritance of the Lord, the charge allotted to them. Compare Deuteronomy 4:20 and 9:29. Biographical note: Philip Schaff was a 19th century church historian who authored, among other works, the 8 volume History of the Christian Church. He was born in Chur, Switzerland in 1819 and died in New York, NY in 1893. He came to the United States in 1844 to teach church history at the Theological Seminary of the German Reformed Church at Mercersburg, PA. In 1863, he moved to New York City, where he worked against the secularization of the Lord's Day, participated in preparing the Revised Version of the Bible (published during the 1880s), and helped to promote a greater awareness of Church history.