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.