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. 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.
… World mission is thus the first and most obvious feature of early Christian praxis.
From The New Testament and the People of God: Christian Origins and the Question of God v. 1 by N. T. Wright.