The following is excerpted from Customs of Primitive Churches by Morgan Edwards (Philadelphia, PA: 1768, p. 45)
PROP. XVI. Of the office of missionaries or evangelists
XVI. The office of missionaries or modern evangelists is, generally, occasional and temporary; therefore their titles (like those of pastors, bishops, &c..) are but relatives, which they lose when the relation that giveth them rise, ceaseth; for which reason they must be chosen out of those that had been ordained teachers. Their work is, to itinerate on the special errands of the churches. They are put in their office by the choice of those churches; ordination; prayer; &c. with fasting. And are to be paid by the same churches.
1. The combination of churches renders itinerant preachers necessary. That there were such in primitive churches appears from 2 Cor. viii. 23, where we read of the messengers or (as it is in the original) the missionaries or apostles of the churches. Also from 3 John. 7. They are the same with the ancient evangelists, extraordinaries excepted.
2. That their business is, to itinerate on the errands of the churches; and that they are to be chosen of the same churches appear, from 2 Cor. viii. 19, Chosen of the churches to travel, &c. Such were the itinerants mentioned in 3 John 7.
3. What the public concerns of combined churches are appear daily. Some are vacant and want supply; some poor that they can get no minister; some have divisions which require help to adjust them; some churches want help in choice, ordinations or settling of officers; often churches are to be [constituted]; some dark corner may want the ministry, &c. How expedient then is the office of apostles of churches!
4. How they are to be put in office is not without some direction in scripture. The office received by Paul and Barnabas at Antioch was, in part, temporary; for in ch. xiv. 26 it is said they returned, having fulfilled the work to which they had been ordained in ch. xiii; and that ordination was by fasting, prayer, laying on of hands; ch. xiii. 2-4.
5. That they are to be paid by the churches which employ them, may be argued from 3 John 7; the ministers there mentioned went forth preaching, taking nothing from the gentiles. Who then did they take something from to bear their expense? why from the churches in Judea who had employed them.
6. If any desire to be further satisfied about the above office, let them read the great Mr. Grantham on the subject, whose method of reasoning is (in our opinion) conclusive. We have seen the ordination of an evangelist or missionary in the north of England; but as we have not minutes of the transaction we can only relate the matter in general. "A set time and place were fixed upon by the churches associating together in that part of the country. Deputies were sent by every church. They met fasting. After common service was over, one of the ministers introduced the business by a short speech, and prayed. Then they consulted who among them was best qualified. Several were nominated; it was put to the vote; which was decided by balloting. He that had the majority of votes was declared duly elected. His acceptance of the office being obtained, he was desired to kneel. The ministers laid their hands on his head; and used words expressive of the action. When ordination was over, the ministers and deputies gave him the right hand of fellowship; and a promise of maintenance in behalf of their respective churches. Then was addressed to him a charge relative to his office, and the duties thereof. A psalm was sung, and the assembly dismissed with the benediction."
[Note: It is interesting that Edwards as a Regular Baptist refers to a General Baptist as "the great Mr. Grantham." He is apparently referring to Grantham's A Defence of the Office of Apostles in 1671 or A Defence of the Office of Subordinate Apostles of Christ in 1678 (apparently much the same material in both works; the latter can be found in Christianismus primitivus: or, The ancient Christian religion, in its nature, certainty, excellency, and beauty (internal and external) particularly considered, asserted, and vindicated, Book IV, Treatise 5, p. 152). Also the reference to Grantham's book and Edwards's attendance of such an ordination might suggest that Edwards's view was not the more common one on this subject among the churches of the Philadelphia Association. On the other hand, the minutes of the association mention them electing at least Morgan Edwards and John Gano to fill the office.]