Regarding "apostle" (APOSTOLOS):
(1) Though in a few instances the word APOSTOLOS does designate simply a leader, in most instances it has the technical meaning of "one sent forth as the highest authority". In Luke 6:13, in the phrase "whom also he named apostles", the word "subsequently" almost certainly is implied, for the Gospel Accounts make apparent the fact that, at this point, the Twelve were far from qualified to exercise authority commensurate with the rank of Apostle. From the point of the sermon of Peter on the Day of Pentecost onward, each of the Twelve (less Judas, plus Paul) was authorized to speak as the vicar of Christ Jesus, with the full authority of Christ Jesus.
(2) The Scripture reveals that the fledgling Church was troubled by false apostles, the most pernicious and conspicuous of whom is James of Jerusalem, the half-brother of the Lord Jesus.
Regarding "bishop" (EPISKOPOS), "elder" (PRESBUTEROS), "pastor" or "shepherd" (POIMEN), and "teacher" (DIDASKALOS):
(1) All four Greek terms refer to the same man; this is the man under whose care the Lord Christ Jesus has placed the sheep of the Local Assembly.
(2) POIMEN is translated "pastor" in Old English and "shepherd" in Modern English.
(3) The responsibility of the Shepherd is enumerated in Psalm 23 and in John 10, but numerous other passages speak of the responsibility and the failing of the men who have functioned in Israel in the role of Shepherd.
(4) The Shepherd answers directly to the Lord Christ, Hebrews 13:17.
(5) The Shepherd does not answer to the deacons of the Local Assembly. Rather, the deacons, like the rest of the sheep, answer to the Shepherd.
(6) The primary role of the Shepherd is to feed the sheep, John 21, Ephesians 4:11-16. In Ephesians 4:11, the conjunction KAI (which the King James Version translates "and") is explanatory, rather than linking; it should be translated "even", for the Pastor IS the Teacher.
(7) In Acts 20:28, the EPISKOPOS is said to have the responsibility of feeding the flock. Thus, it is apparent that the DIDASKALOS and the EPISKOPOS are one and the same.
(8) The means by which the sheep are fed is instruction in the Scripture, Matthew 4:4, Luke 4:4, Amos 8:11-12.
(9) In addition to feeding of the sheep, the responsibility of the Shepherd is to oversee the flock; thus, he is the EPISKOPOS. Oversight may involve exhortation or reprimand.
(10) The term PRESBUTEROS does not necessarily signify age; it is a term of respect which indicates authority; this is the case with the so-called "Elders of Israel" mentioned repeatedly in the Gospel Accounts and the Book of Acts. In I Timothy 5:17, elders are seen in the role of ruling (which is the role of the POIMEN) and labouring in the Word and doctrine (which is the role of the DIDASKALOS). In Titus 1:5, the ordination of elders (PRESBUTEROS) in every city obviously is the ordination of men to the office of leader or ruler of the Assembly. Thus, PRESBUTEROS is used figuratively of the Shepherd of the Local Assembly; he is the "Old Man".
(11) The work of the Shepherd is a full-time job, consider Acts 6:2-4. The Lord Jesus has ordained that the Shepherd, as one who proclaims the Gospel, receive his sustenance from the Gospel, I Corinthians 9. Even as it is the responsibility of the Shepherd to supply the spiritual needs of the sheep, so also it is the responsibility of the sheep to supply the physical needs of the Shepherd.
(12) The tithe, if faithfully paid, enables a Local Assembly consisting of as few as ten families to support the Shepherd and his family at their own standard of living. And though the tithe seldom is paid faithfully, it provides a reasonable starting point.
(13) With respect to giving, the contemporary Protestant Pulpit typically cites II Corinthians 9:7. However, anyone who takes the trouble to read chapter 9 in its entirety is aware that that verse 7 has to do with the one-time special monetary gift which Paul was soliciting for relief of the impoverished Christians in Jerusalem. Contribution to that gift was optional -- not grudgingly or of necessity. But such is not the case for support of the Shepherd of the Assembly.