They were commissioned:
As they ministered to the Lord, and fasted, the Holy Ghost said, Separate me Barnabas and Saul for the work whereunto I have called them. And when they had fasted and prayed, and laid their hands on them, they sent them away.
We witness here a commissioning service in Acts 13, a laying on of hands. This wasn’t ordination, but a formal recognition and separation for a special task.
The Apostle Paul was already a missionary, but now the Antioch church gives him formal recognition and authority unto this new task. Acts 13 wasn‘t Paul‘s ordination service, but a formal declaration that he was to be sent forth with a mission. Such an act confirms the local church‘s commendation of the missionary. It is their seal of approval, a transfer of authority, granting the missionary the right to act in the name of the church for the sake of the Glory of Jesus.
When a church lays on hands this is a testimony that they recognize the fittedness and the preparedness of the missionary to serve in that cross-cultural capacity for which they were commissioned.
It is an affirmation of suitability and, therefore, not a light or casual event. As eager as local churches are to send one of their own to the field, such a serious step should give pause to churches lest they risk turning their ugly ducklings into swans and confirm one who should not be sent. Many commissioning services include a charge both to the missionary and also to the sending church body, reminding them of their mutual obligations.
Such a laying on of hands is an evidence that the missionary is not merely one who runs forward on his own, but is one who is sent. He is not laying hands on himself, but the larger body of Christ is testifying that the missionary is truly, indeed, a “sent-out one.”