Dear Brother,
Barnabas wasn't chosen to be an apostle, only to go with Paul on his missionary work--he was already an apostle. Timothy is never called an apostle--Paul calls him a 'brother' and 'co-worker' (Colossians 1:1, 2 Corinthians 1:1, Romans 16:21) Same with Silvanus (1 Peter 5:12)
"But when the apostles Barnabas and Paul heard of it, they tore their robes and rushed out into the crowd, crying out"
--Acts 14:14
To be an apostle, a disciple must meet a certain set of criteria;
1. They must be commissioned by the Lord Himself to spread the gospel before His ascension
2. They should show evidence of their apostleship by the working of miracles
3. They must have been EYE WITNESSES to his miracles, ministry, and resurrection.
There is no one currently alive that meets this criteria.
Barnabas was chosen by the Holy Spirit to be sent with Saul as an apostle. He was first a prophet or a teacher as was Saul.
"Now in the church that was at Antioch there were certain prophets and teachers: Barnabas, Simeon who was called Niger, Lucius of Cyrene, Manaen who had been brought up with Herod the tetrarch, and Saul.
During a time of fasting a praying, the Spirit instructed them to set apart Barnabas and Saul.
"As they ministered to the Lord and fasted, the Holy Spirit said, “Now separate to Me Barnabas and Saul for the work to which I have called them.”
The Elders acknowledge the work of the Spirit, lay hands on them, and send them out. Then this is recorded:
"So, being sent out by the Holy Spirit, they went down to Seleucia, and from there they sailed to Cyprus."
Ultimately, they were sent out by the Holy Spirit. The Spirit chose them. And the definition of "apostle" is "sent one". Barnabas did not meet the criteria set by Peter and the disciples. (I'll come back to that)
Regarding Timothy and Silvanus.
In the introduction of the first letter to the Thessalonians this is written:
"Paul, Silvanus, and Timothy,
To the church of the Thessalonians in God the Father and the Lord Jesus Christ:
Grace to you and peace from God our Father and the Lord Jesus Christ."
These are the authors of the letter.
It continues:
"We give thanks to God always for you all, making mention of you in our prayers..."
Who is the "we"? Clearly it is Paul, Silvanus, and Timothy. It continues...
"And you became followers of us and of the Lord..."
Who is the "us"? Clearly it is Paul, Silvanus, and Timothy. It continues...
"..that our coming to you was not in vain."
Who is the "our"? Clearly it is Paul, Silvanus, and Timothy. It continues...
"But as we have been approved by God to be entrusted with the gospel..."
Again, the "we" is Clearly it is Paul, Silvanus, and Timothy.
"For neither at any time did we use flattering words, as you know, nor a cloak for covetousness—God
is witness."
This is a letter written by three men, Paul, Silvanus, and Timothy. So it is written from the perspective of three men addressing the saints in Thessaloniki.
The next verse is this:
"Nor did we seek glory from men, either from you or from others, when we might have made demands as apostles of Christ."
Who are the apostles of Christ? Paul, Silvanus, and Timothy
Regarding the standard set forth by Peter and the apostles. This is said:
“Therefore, of these men who have accompanied us all the time that the Lord Jesus went in and out among us, beginning from the baptism of John to that day when He was taken up from us, one of these must become a witness with us of His resurrection.”
This standard was necessary because of the legal traditions of the Jewish people. With the witness of 12 Jewish men a matter was considered irrefutable. And, in the Lord's timing at Pentecost, they first witnessed to the Jews. All 12 stood up with Peter as he delivered the message. By tradition, this was a sign that all 12 agreed with the one speaking. Peter then gave an account of Jesus' ministry, his death and resurrection, and ascension.
Do you see it now?