All Christians are missionaries

So that they might be with him and he might send them out to preach. Mark 3, 14-15)And Jesus went up on the mountain and called to him those whom he desired, and they came to him. And he appointed twelve (whom he also named apostles) so that they might be with him and he might send them out to preach and have authority to cast out demons. He appointed the twelve: Simon (to whom he gave the name Peter); James the son of Zebedee and John the brother of James (to whom he gave the name Boanerges, that is, Sons of Thunder); Andrew, and Philip, and Bartholomew, and Matthew, and Thomas, and James the son of Alphaeus, and Thaddaeus, and Simon the Zealot, and Judas Iscariot, who betrayed him. (Mark 3: 13-19)

Right from its very beginning, Christianity is intrinsically a missionary faith. Jesus immediately sent His apostles to preach the Gospel. Even today, Christians must proclaim salvation to those who are baptised in Jesus' name and then to others, who whilst God's creature, are not yet baptised and therefore not yet children of God or brothers and sisters in the Christian faith. Every Christian, thus, is a missionary and every missionary must preach the Word of God and not his own ideas.