Separated to the Work

Acts 12:25-13:3 
25 And Barnabas and Saul returned from Jerusalem when they had fulfilled their ministry, and they also took with them John whose surname was Mark. 
1 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. 2 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.” 3 Then, having fasted and prayed, and laid hands on them, they sent them away.

Barnabas and Saul were ministering God's word when called to something more specific by God through the hands and assent of others. So now God moves elders to appoint preachers, elders, deacons, and other ministers to serve in the gospel to be sent into the harvest.  This could be on missions, as Saul and Barnabas, or as the elders in churches everywhere.  The pattern is God's call, the commissioning, and the sending into service to minister the words of life.