According to Luke's Acts of the Apostles, the last thing Jesus did before he bodily ascended to heaven was to commission the disciples to 'witness' to his teachings. 'Disciple' meant 'student' and was derived from the various schools of philosophy in the ancient world. There was a master/teacher, and his students collected the teachings and passed them on. From this point forward in the text, however, the term 'disciple' is often replaced by 'apostle'. 'Apostle' (from the Greek 'apostellein' meaning "one sent out") was a herald. The Latin for 'apostle' was 'missio', from which we derive our word 'missionary'.
This is a companion discussion topic for the original content at https://www.worldhistory.org/article/1658/the-first-christian-missionaries/.