The active, expansive missionary work among Jews and Gentiles carried out by the early Christian movement was a shocking innovation in antiquity.
The spread of the Christian movement was not a fluke. Jesus intentionally founded an international missionary movement.
In just forty years the Christian missionaries had planted churches in the Roman provinces of Syria-Cilicia, Cyprus, Galatia, Asia, Mysia, Macedonia, Achaia, Cappadocia and Pontus-Bithynia; in Italy and Rome; in Dalmatia; on Crete; possibly in Illycrium and Egypt. Churches were planted in the major cities of the Roman Empire—Jerusalem, Damascus, Caesarea, Antioch, Rome, Corinth, Alexandria, and Ephesus. Those churches served as bases for outreach into the surrounding regions.
There's credible evidence that the apostle Thomas made it to India. Proof for Christians in India dates to the third century.
The early Christians were following the example set by Jesus to reach all the people in a region, whether they lived in cities or small settlements, regardless of social status or religious background.
How can we explain this "shocking innovation"? Without the life, death and resurrection of Jesus, without the coming of the Holy Spirit upon his followers, it makes no sense at all.
There's a restlessness about movements. Any movement that settles down ceases to be one.