The principle of multiplication

Considering that the church prior to Pentecost numbered only a few hundred believers, this is an astounding achievement. Probably the Christian community within three decades had multiplied four hundredfold,18 which represents an annual increase of 22 percent for more than a generation, and the rate of growth continued remarkably high for 300 years. By the beginning of the fourth century, when Constantine was converted to Christianity, the number of disciples may have reached 10 or 12 million, or roughly a tenth of the total population of the Roman Empire.
Such growth cannot be sustained by merely adding the children of Christians to the rolls, nor is it the result of large transfers of membership from other congregations. The early church grew by evangelistic multiplication as witnesses of Christ reproduced their life-style in the lives of those about them.

It does not matter how small the group is at the beginning, provided that they implant their vision in men and women who will in turn pass the word along to others and that they also reproduce. The early church gave eloquent witness to the dynamic in the hearts of people who take God at His Word and believe that with Him nothing is impossible.

Robert Emerson Coleman, The Master Plan of Discipleship (Old Tappan, NJ: Fleming H. Revell Company, 1987), 39–40.