During the time of the ancient Greek orator Demosthenes (384-322 BC), the word apostolos was a naval term that described an admiral, the fleet of ships that traveled with him, and the specialized crew who accompanied and assisted the admiral. In the ancient world an apostle was the personal representative of the king, functioning as an ambassador with the king’s authority and provided with credentials to prove he was the king's envoy.
Apostolos was a technical word designating an individual sent from someone else with the sender's commission, the necessary credentials, the sender's authority and the implicit responsibility to accomplish a mission or assignment.
ἀπόστολος apóstolos, ap-os'-tol-os; from G649; a delegate; specially, an ambassador of the Gospel; officially a commissioner of Christ ("apostle") (with miraculous powers):—apostle, messenger, he that is sent, Or a sent One
When we are sent out by our local congregation we are on an apostolic mission, and we can consider ourselves "Apostles" because we were sent with a specific mission.
Jesus gave us this mission in Matt 28:19 when He said:" go into all the world and disciple all nations" This is an apostolic mission. To bring what is in heaven to earth. To rule and reign on this earth the way the Lord Jesus Christ showed us.