A great quote from Tom Wright’s The New Testament and the People of God,
Why then did early Christianity spread? Because early Christians believed that what they had found to be true was true for the whole world. The impetus to mission sprang from the very heart of early Christian conviction. If we know anything about early Christian praxis, at a non- or sub-literary level, it is that the early Christians engaged in mission, both to Jews and to Gentiles. ‘The irresistible expansion of Christian faith in the Mediterranean world during the first 150 years is the scarlet thread running through any history of primitive Christianity. [quoting Hengel] This missionary activity was not an addendum to a faith that was basically ‘about’ something else (e.g. a new existential self-awareness). ‘Christianity was never more itself than in the launching of the world mission.’ [quoting Meyer]
Put simply, mission is part of the DNA of the people of God. Does my life reflect this? Does yours? How about your church? To stretch a metaphor, is mission the skin we are in or a fashion accessory we put on from time to time?