History of the Church: A. D. 100 to 311 —
Clergy and Laity
Philip Schaff
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.