Christians, 40-350 CE
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Jesus compared the kingdom of heaven to a mustard seed, "which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs" (). The sociologist Rodney Stark, in The Rise of Christianity (1996), modelled how the church grew from a small band of believers into a large share of the Roman world. Starting from 1,000 Christians in 40 CE and growing at "40 percent per decade" (about 3.4% a year), his table reaches 7,530 by 100, 217,795 by 200, 6,299,832 by 300 (10.5% of the empire) and 33,882,008 by 350 (56.5%). Stark's point is that steady growth, kept up over generations, is enough to carry a tiny movement to a majority of the empire. The table, as reproduced by Ian Paul, shows the mustard seed becoming the greatest among herbs.
Model: these are not counts. No count of Christians survives from any of these dates. The table applies one constant growth rate between a guessed starting point (1,000 in 40 CE) and an estimated anchor (about 6 million in 300). The New Testament scholar Bart Ehrman, summarising Stark, puts the needed rate at "43% per decade". The model's own point is that the end figure needs no extraordinary rate of growth.
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Because the figures are a model, they cannot show how fast the church actually grew in any particular decade. The starting figure is a guess: Stark himself treats it as low, and both he and Ehrman read the "thousands" in Acts, "about three thousand souls" () and "about five thousand" (), as an exaggeration. If the true start was larger, or the figure for 300 smaller, the rate needed is lower still. The 350 figure is the model's projection, not an independent estimate.