From the coming of Christendom through a period extending for more than a thousand years, most of the leaders of the Christian faith at one time or another carried the title of "priest." This continues today in the Catholic and Eastern Orthodox branches of Christianity. I do not want to disparage those individuals, many of whom were and are faithful followers of Christ. It would have been nearly impossible for them to serve without being set apart to a unique calling as a priest. When I was a youth, there was a priest who treated me with a kindness and Christian maturity that I did not exactly reciprocate. I still have a Bible he gave me. So I wouldn't want what I am about to write to be misunderstood as an expression of hostility toward anyone who is a priest. It's just that unfortunately, the creation of the priest as a church officer was a major error.
A priest is an individual who serves as a mediator between God and man. In a religion with priests, it is not possible for a regular person to have direct access to God; it is necessary to go through the priest, or at least to have the priest perform some service to allow access. This role of the priest is well-defined in the Old Testament and was practiced in Judaism at that time (though not today) and is also reflected in many other religions in the world, including pagan and folk religions.
However, the New Testament teaches that all believers are priests (1 Peter 2:4-9, Revelation 1:6, 5:10, 20:6). It teaches that Jesus himself is the Great High Priest who acts as a mediator between God and man and through whom man has direct access to the throne of God (Hebrew 4:14-16). The New Testament does not envision any role for a human priest as a church leader; church leaders were given different titles, such as elders, pastors, apostles, etc. but not priests.
If it was merely a matter of a title it would not have been so bad, but priests under Christendom eventually did begin to serve in a role as a mediator between God and man, and this was a very bad thing. Forgiveness of sins required giving a confession to a priest. Rather than repentance, a priest could assign someone to "do penance." Eventually, the practice of indulgences arose, where forgiveness could be bought with money.
The designation of some individuals as priest also led to a sharp hierarchy in the church which even many protestants that shook off the concept of priests have not been able to avoid. The church leaders became professional Christians, while the laity were merely amateurs. The amateurs did not feel required to be as committed as the professionals: they did not need to learn the Bible or practice the more advanced functions of the professional members of the faith. Conversely, professionals will usually not feel there is anything to learn from amateurs. This amateur/professional division was a factor in curtailing the missionary imperative that initially drove the spread of the faith.
The Protestant Reformation, beginning in 1517, served to undo some of the harm. In fact, the reformation even helped reform the Catholic churches to an extent, as indulgences and some other excesses are no longer practiced in Catholicism.
It is not certain how much of a role Christendom itself served in the rise of the priests. Perhaps this was a historical development that would have happened even outside of a state church and its coming was just coincident with the rise of Christendom. But whether or not Christendom was the cause or whether it just happened at the same time, the office of a priest was a mistake.
