32 Likewise all know that the Gospel followed the great Roman roads and was spread not only by Bishops and priests but also by public officials, soldiers and private citizens. Thousands of Christian neophytes, whose names are today unknown, were fired with zeal to promote the new religion they had embraced and endeavored to prepare the way for the coming of the Gospel. That explains why after about 100 years Christianity had penetrated into all the chief cities of the Roman Empire.
Source: Evangelii Praecones (Vatican.va)