Christ is the Head of the Church and source of all its authority. He rules the Church, not by force, but by His Word and Spirit. All human officers in the Church are clothed with the authority of Christ and must submit to the control of His Word.
The officers of the Church mentioned in the New Testament are of two kinds:
a. Extraordinary officers. The most important of these were the apostles. In the strictest sense this name applies only to the Twelve chosen by Jesus and Paul, but it is also given to some apostolic men, such as Barnabas.
The apostles had certain special qualifications. They were directly called by Christ, saw Christ after the resurrection, were conscious of being inspired, performed miracles, and were richly blessed in their labors.
In Ephesians 4:11, Paul also mentions EVANGELISTS, who assisted apostles in their work.
b. Ordinary officers. Frequent mention is made of ELDERS, especially in the Acts of the Apostles, Acts 11:30; 14:23; 15:2, 6, 22; 16:4; 20:17; 21:18.
Alongside of it the name ‘bishop’ was used to designate the same kind of officers, Acts 20:17, 28; 1Tim. 3:1; 5:17, 19; Tit 1:5,7; 1 Pet. 5:1-2. While both names were applied to the same class of officers, the name ‘elder’ stressed their age, and the name ‘bishop’ their work as overseers. The elders were not originally TEACHERS, but gradually the teaching function was connected with their office, Eph. 4:11; 1Tim. 5:17; 2Tim. 2:2. From
1 Tim 5:17 “Let the elders that rule well be counted worthy of double honor, especially those who labor in the word and in teaching.”
It appears that some elders simply ruled, while others also taught. In addition to these the New Testament also speaks of DEACONS, Phil. 1:1; 1Tim 3:8, 10, 12. The institution of this office is recorded in Acts 6:1-6.
[From Berkhof Summary of Christian Doctrine]