When discussing the issue of collecting funds for the poor saints in Jerusalem, the apostle Paul mentions three men. One of them is Titus, who the Corinthians were already familiar with. The other two are praised, but they are not named.
Thanks be to God, which put the same earnest care into the heart of Titus for you. For indeed he accepted the exhortation; but being more determined, of his own accord he went to you. And we have sent with him the brother, whose praise is in the gospel throughout all the churches. And not that only, but who was also chosen by the churches to travel with us with this grace … And we have sent with them our brother, whom we have many times proven diligent in many things, but now much more diligent, on the great confidence which he has in you. Whether any inquire of Titus, he is my partner and fellow-helper concerning you. Or if any inquire of our brethren they are the apostles of the churches, the glory of Christ. (2 Corinthians 8:16-19, 22-23).
First, since Paul doesn’t mention the names of either of the two men who went with Titus, it is impossible to know with absolute certainty who is under consideration. Having said that, there are some intriguing possibilities. The two brethren are called “messengers of the churches” (KJV) or “ambassadors of the congregations” (MLV). The Greek word is apostelloi, which means “apostles.”
The first brother is one who was very well-known. In fact, Paul says that this brother had praise in the gospel throughout all the churches. The most likely candidate is Luke, the beloved physician, who had written his gospel account, which had spread throughout all the congregations of the Lord’s church. According to Coffman, “some of the oldest traditions affirm” that Luke is this first brother.
But the gospel written by Luke was not what caused him to be called an “apostle.” It was the fact that he was “chosen by the churches” to accompany Paul on this mission of collecting funds for the poor saints in Jerusalem. The original language indicates that the congregations gave a show of hands or raised their voices in support of sending Luke (or whoever it might have been) with Paul. This shows that (1) he was respected, (2) he was trusted, and (3) that the churches thought the mission was a worthy one. Luke didn’t just have the support of one congregation, but all the congregations in that area chose him to go. He was a representative on a mission for the churches of an entire area. That speaks volumes about who he was and about the unity that the churches had with each other.
The second brother that Paul calls an “apostle” of the churches is unknown to us, though some have suggested different names from among Paul’s letters. What is known about him, though, is that he had proven himself to be faithful and dependable time and time again in everything that the churches (and Paul) had asked of him. He was diligent, he kept working, and now he’s even more diligent because of the mission that is set before him and his confidence in its successful completion. Is it any wonder that the congregations chose this man to accompany Titus and Luke?
These men are called “apostles of the churches,” but then Paul adds the words “the glory of Christ.” It could be that Paul is saying that those who do the work of the Lord are bringing glory to Christ. How much of a greater commendation could there be? Imagine that on the day that you are judged, you hear the words “You brought glory to me.” These apostles of the churches brought glory to Jesus Christ, as should we.
-Bradley S. Cobb
 This suggestion is given by Patterson (I and II Corinthians, pages 56-57), Barnes, Clarke, Coffman, Coke, Henry, Lipscomb, McGarvey, Robertson, and others. Henry Samuel Baynes in his Horae Lucanae: A Biography of Saint Luke (pages 197-212) presents a very compelling case that Luke is the first brother mentioned in 2 Corinthians 8.
 In favor of Luke as this first brother, it should be noted that he joined Paul in Macedonia (where Philippi is located), and then when they arrived back at Philippi later on, he stayed there. He again joins with Paul the next time he passes through Philippi, on his way to Jerusalem. Notice the change in pronouns from “they” to “we” throughout the book of Acts (see especially Acts 16:12, 20:6).
 Acts 20:4 mentions several traveling companions of Paul, some of whom were from the Macedonian region. It may be that the brother mentioned in 2 Corinthians 8:22 is one of these men. If this is the case, Aristarchus seems the most likely, as he is mentioned by Paul in Colossians and Philemon. Others have suggested Barnabas or John Mark, though there’s no evidence that either of these men were ever members of Macedonian churches (and thus couldn’t be sent out by them).