Did the early christian church grow very fast?


Let’s say at the start that this isn’t the most important question in the world! But I think it is interesting.

The background is this. The New Testament (Acts 1:15) records there were 120 disciples in Jerusalem a few weeks after Jesus’ death and resurrection. Three centuries later, historians estimate that there were somewhere between 3 and 8 million christians in the Roman Empire.

Keen to show God’s hand in their beliefs, christians have been known to point to this massive growth, while some sceptics are keen to downplay it. I believe both sides have overstated their case. If you’re interested in history, or if you are a maths nerd, read on! If not, I won’t mind if you leave quietly now. πŸ™‚

He said, she said, they said ….

Sociologist Rodney Stark was apparently the first person to look at this question using a simple mathematical “model”. (I can hardly believe that no-one thought of doing this before him, but that’s what they say.) It’s dead easy with a spreadsheet. Just set up a column for population that grows a certain percentage each year or decade, and you’ll soon find that, depending on your starting number, a growth rate of 40%-50% per decade or 3.5%-4% per year is enough to get to about 6 million in 300 CE.

This is a healthy growth rate, but hardly miraculous. And so people started making comparisons, and suggesting, for example, that the early christians had done no better than Mormons have done in modern times. Some argued atheism had grown faster in the 20th century, which showed that “the gods are losing”.

Growth rates, conversion rates and statistics

As someone who enjoys playing with spreadsheets (and used to use them often in my work), I was immediately a little suspicious about these conclusions. Intuitively, it seemed to me that these assessments confused two very different statistics.

Growth rate

Growth rate is a factual piece of data, but it doesn’t necessarily give a good picture of the “success” of a movement, and cannot readily be compared across time periods – because growth rate is dependent on several factors: the conversion rate, but also the birth rate and life expectancy. With birth rates in the first few centuries many times higher than now and life expectancy at about a third of what it is now, these demographic factors could make an enormous difference to the growth rate.

Conversion rate

The conversion rate is a much better indication of the “success” of a movement, but cannot be obtained from data alone – it requires a mathematical model to calculate it.

Theory and reality

I decided to test my intuitions using my trusty spreadsheet.

I started with a simple hypothetical model, which showed that if we halve the birthrate and double life expectancy, but keep the same rate of conversion, a movement’s growth rate approximately doubles. Or put it another way, a modern (20th century) movement only requires three quarters of the conversion rate to achieve the same growth rate as an ancient movement.

Then I constructed models of 270 years of early church growth and 140 years of Mormon growth using the best data I could find on the appropriate birth rates and life expectancy. However I found that comparisons of conversion rates were not reliable, for several reasons:

  • The demographic data (population numbers, birth rates and life expectancy) and not well known in the Roman Empire, and often vary significantly over time and place, making any simple model too simplistic.
  • The final growth and conversion rates were very dependent on the starting population chosen. Both movements appear to have started with very fast growth, and using the very earliest starting number gave very different answers to when I began a little later when the numbers were better known.

Atheist growth rate

At the now dormant Commonsense Atheism site, Luke’s claim for a a high rate of growth in global atheist numbers was easier to analyse, and proved to be quite problematic.

  1. The group he identified as “atheism and agnosticism” is actually more accurately described as “non-religious” or “unaffiliated”. About half the unaffiliated actually have some belief in a god or life force, so the numbers of atheists and agnostics are probably about half those he quoted. About three quarters of these are agnostics and a quarter atheists.
  2. Many graphs of 20th century religious statistics show the numbers of unaffiliated rising significantly in the first half of the century and then falling again in the second half. A major reason for this is the counting of people in communist countries as non-religious and the the return of many to religion when communism fell in Russia and eastern Europe. At present, the global population of unaffiliated is increasing slowly, below population growth, so it is declining in percentage terms, and the global number of atheists is declining in absolute terms.
  3. The growth of early christianity was against a background of very slow population growth in the Roman Empire, whereas the growth of the non-religious in the 20th century was on top of much larger natural population growth. When this is taken into account, the conversion rates are not very different, and the non-religious rate was only sustained for about 50 years.
  4. As I found for the early christian and Mormon statistics, the starting point makes a lot of difference to the final answer. Numerical comparisons are very arbitrary.


  1. The strongest conclusion is that all estimations and comparisons are unreliable because the data accuracy is problematic, “success rates” depend on birth rates and life expectancy, which vary greatly over time, and the choice of starting point can be arbitrary and make a large difference in the final answer. All sides would do well not to claim too much.
  2. The growth of the early christian church over 3 centuries was impressive but not unprecedented. The growth of the Mormon church in the 19th and 20th centuries, and the growth of those not affiliated with any religion in the 20th century, were not very different when the underlying population demographics are taken into account. However the growth of the unaffiliated was only sustained for about 50 years, far less than the christian and Mormon growth was sustained.
  3. Globally, atheists comprise only about an eighth of the unaffiliated, and their numbers are not growing at present.
  4. The success or otherwise of any belief system or worldview says little about its truth or otherwise.
  5. Exercises like this can consume a lot of time (this information took me many, many hours to compile and analyse) for very little productive result. πŸ™

Further reading

Photo Credit: griseldangelo1 via Compfight cc.


  1. There is an even bigger fallacy in people using these numbers for claiming early Christianity underperformed compared to Mormonism. These figures are for Christianity within the Roman Empire, but we know for a fact that there were also Christian communities outside the Roman Empire by the fourth century. There was definitely one in Persian lands (Mani used to be a Christian!) and there was also a (maybe rather) small one in India.

    It is an undercount, but I have no idea by what margin. I should add that I’m not claiming that it is Stark’s claim that this accounts for all of Christianity in the fourth century.

  2. Hi, yes I thought of that, but I thought the numbers were probably relatively small, within the margin of error of the other numbers anyway. It was only when I spent quite a while trying to build spreadsheets that reproduced the claimed population and christian/Mormon/non-religious growths (based on birth rates and mortality rates for different ages), that I realised how dodgy the numbers often are and how a few changes can make an enormous difference in the outcome. I think Stark uses the numbers responsibly, but others with stronger agendas don’t always.

  3. Well, considering that before the Council of Nicaea Armenia adopted Christianity as the state religion, that later in the fourth century there was a Ghassanid Arab kingdom that was strongly Christian, that centuries later Mesopotamia housed a church with millions of followers and that there was already a probably small church in India at the time, I would lean to a higher estimate, though it is surely difficult to put anything definite on it.

  4. Interesting, do you factor in strength of opposition? Certainly the mormons and atheists did not have nearly the opposition as the early church. The early church was under extreme persecution from both Rome and the Jews?

  5. Hi Darby, I’m sure that is right. I was just dealing with the figures (as best as I could estimate them), but when we try to interpret the data, that would be a factor.

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