The number of Christians grew by approximately 40% per decade during the first and second centuries. In the post-Apostolic church a hierarchy of clergy gradually emerged as overseers of urban Christian populations took on the form of "episkopoi" (overseers, the origin of the terms bishop and episcopal) and "presbyters" (elders; the origin of the term priest) and then "deacons" (servants). But this emerged slowly and at different times in different locations. Clement, a 1st-century bishop of Rome, refers to the leaders of the Corinthian church in his epistle to Corinthians as bishops and presbyters interchangeably. The New Testament writers also use the terms overseer and elders interchangeably and as synonyms.