Let’s summarize how Luke describes the process of Barnabas and Saul leaving the church of Antioch at the beginning of Acts 13.  The Spirit sets them apart, v.2, then the church releases them, v.3, and finally the Spirit is said to have sent them out, v.4.  The church does not primarily set apart men for gospel ministry, but the church does formally recognize men whom the Holy Spirit has set apart and then sends them out.  Even extremely gifted men like Barnabas and Saul are sent out by the church after having hands laid upon them.  So just like Acts 6 laid the groundwork for the office of deacon, even though those men were not technically deacons, Acts 13:2-3, lays the groundwork for missionaries, even though Saul and Barnabas are apostles first and foremost, which included a missionary function.  So the basic definition of a missionary then is one who is set apart for gospel ministry by the Holy Spirit and confirmed by the qualified leaders of a church.  That basic definition protects us from so much erroneous thinking in the category of missions.