That is my inference, though it is quite reasonable. Of course, as Stark readily acknowledges, Christians in this period did engage in evangelism. But not to the degree that we might assume given the statistics of growth he marshalls. In our present day church growth 'culture' we naturally think that growth in church membership is the result of a definite strategy. But this seems not to have been the case.
Strategy appears to have been lacking and evangelism haphazard and sporadic.
The answer, as I read Stark, was that their secret was conviction about what they believed which they fearlessly expressed in the face of hostility. Theirs was a certainty of hope based on the resurrection of Jesus in face of disillusionment among the general populace about the gods of Greece and Rome. The Christians' beliefs and practice, however, were not held in a historical vacuum. In AD 165 and 250-260 two dreadful epidemics swept through the Empire, in each case wiping out up to half the population. Unhygienic and stinking places at the best of times cities like Alexandria reeked with dead and diseased people.