The word 'fellowship' is a new covenant word. The fellowship spoken of in the new covenant is patterned after the fellowship that Jesus and the Father had with each other during Jesus' earthly days. Jesus' prayer was that the fellowship among His disciples would be of the same order. Under the old covenant, although people could rise to great heights of holiness, yet they could not come into fellowship with each other. There were godly men under the old covenant - Moses, Elijah, Daniel and John the Baptist, to name but a few. These men had a holiness that exceeded the holiness of most of today's believers. But that is because most of today's believers have not entered into the new covenant.
The new covenant leads the disciples of Jesus into an inward sanctification, that in turn leads to fellowship one with another. When we read of the great men of faith in Heb. 11, we see that they were all lonely individuals. This is how it was in Old Testament times. But as soon as we turn to the New Testament, we find Jesus sending out His disciples two by two. This was something new. Jesus came not only to lead us to an inward sanctification but also to fellowship. If a believer comes to a life of victory over sin inwardly, and yet does not come into fellowship with others, there is something drastically lacking in his sanctification. Sanctification without fellowship is a deception. Many are travelling around the world today, preaching holiness; but they themselves are lonely individuals like those in Old Testament times. Such preachers are still under the old covenant. Invariably it will be seen that they have not built any fellowship in the place that they reside.
But it was not so with the apostles in the first century. Soon after the day of Pentecost, we read of Peter and John going out together. Peter told the lame man in the temple to look at both John and himself (Acts 3:4). Peter and John worked as a team. On the day of Pentecost, even though it was Peter who preached, yet we read that he stood up with the eleven (Acts 2:14). Fellowship is the one thing that stands out when we read Acts chapters 2 to 4. Peter and John were not men of similar temperament. They were vastly different as human beings. Peter was the quick and active type - quick to boast that he would never deny the Lord, quick to jump into the sea of Galilee, as soon as he saw the Lord by the shore (Jn. 21), etc., John, on the other hand, was the quiet meditative type who loved to be alone and to see visions of heavenly things (as at Patmos). God always brings together people who are dissimilar (humanly speaking), in the church - so that He can demonstrate a unity in diversity that is far more glorious than the unity of two similar people becoming one.
In Acts 13:2, we read of the leaders of the church in Antioch fasting and worshipping the Lord, seeking His direction. The Holy Spirit then spoke to them to separate Saul and Barnabas for His service. Notice again, that unlike Old Testament times, the Spirit called two people and not one. This was the new covenant age and there was no place now for an individualistic ministry. There had to be an expression of Christ's body - and for this, a minimum of two people were required. Here again the Spirit called two people of dissimilar temperaments to work together. Paul was a strict, uncompromising man who would not tolerate any halfheartedness in anyone. In Acts 15:36-39, when Paul and Barnabas had a discussion concerning asking Mark to accompany them on their second journey. Paul would have none of it, because Mark had left them half way through their first journey. Barnabas, (who was given that name because he had such an outstanding ministry of encouragement - Acts 4:36) on the other hand, wanted to give Mark one more chance. Paul and Barnabas stuck so much to their own view points on this issue that they separated from each other. Obviously, they were both still very strong-willed and had not yet come (in their own spiritual development) to the place where they had acquired the wisdom from above that is 'willing to yield' (Jas. 3:17).
The apostles were not ready-made saints. They too had to develop like all of us - and get light on their flesh, little by little. Later on Paul, Barnabas and Mark came into a glorious fellowship with each other (as is evident from 2 Tim. 4:11).
Paul and Barnabas had dissimilar temperaments. The Holy Spirit had called them together. Yet they did not know how to get along with each other. This is the condition with many believers even today. Such a condition is tolerable among the immature. But what shall we say when we see such a condition existing even among those who have been believers for over ten years. That is pathetic, to say the least. Paul majored on 'truth'. Barnabas majored on 'grace'. If each had seen and appreciated the need for the other, the glory of God could have been seen in them, full of grace and truth (Jn. 1:14). Together they could have produced something that neither of them could have done individually. This is why the Holy Spirit called them together. If a church had only the ministry of a man like the young Paul, everyone would have been driven out, and only Paul would have been left! If, on the other hand, it had only the ministry of a man like the young Barnabas, it may have ended up as a jellyfish-like organisation without backbone, and full of halfhearted compromisers. But together, Paul and Barnabas could have built the true church. This is what Satan did not permit them to see in Acts 15. Thank God that they saw it later.
When Barnabas left Paul, the Holy Spirit prepared another co-worker for Paul. We read of this person immediately after the Acts 15 incident, in Acts 16:1 - Timothy. The Spirit was not going to let Paul minister alone. He prepared a co- worker for him, who again, was temperamentally the exact opposite of Paul. Timothy was a shy, retiring, timid type of person - an introvert, in contrast to Paul the extrovert. They were both totally alike in their devotion to the Lord, in their wholeheartedness, and in their freedom from 'seeking their own' (Phil. 2:19-21). But they were poles apart temperamentally. Yet Paul grew to appreciate Timothy more than any of his other co-workers. They had glorious fellowship together, despite their different temperaments. At last the Holy Spirit was able to accomplish what He wanted to, in Paul.