You may have heard of exponential growth, and, if so, you probably think of it as something that grows really, really fast.  The most commonly given example is bacterial growth.  Bacteria reproduce by division, and in any fixed length of time, a certain percentage will divide.  If you put a small colony of bacteria in a petri dish (with the appropriate nutrients), at first the population grows slowly, because there are only a few bacteria present to divide. But as the population grows, there are lots available to divide, and the population grows faster and faster and faster, until eventually they run out of food and space.  Other examples of exponential growth are given by nuclear chain reactions, compound interest, the spread of infections, and fraudulent pyramid schemes.  Note that the fast growth of the bacterial population doesn’t come from each of them dividing into millions of pieces.  Hold that thought.

            In 1996 the sociologist Rodney Stark published the book The Rise of Christianity, which gave reasons for the rapid spread of early Christianity.  In one part of the book, he estimated the number of Christians at various dates.  These data were tweaked in Bart Ehrman’s 2018 book The TRIUMPH of CHRISTIANITY, andI’m giving his data below.  I’ve looked up the estimates of other authors, and while they all differ, they don’t change the basic picture.

The year 312 is the year of Constantine’s Edict of Milan giving Christians religious freedom to people of the Roman Empire, including Christians. 

            Stark and Ehrman have hundreds of pages devoted to giving reason explaining the growth of Christianity, but we can learn a lot just from these data, especially the average annual growth rate.  Ehrman’s data give an annual growth rate of about 3.3%, which I suspect you find surprising low.  Some historians give a value of 6,000,000 Christians during the time of Constantine, but this higher value still only yields an average annual growth rate of 3.9%.  This says that a congregation of 100 would typically add a net of 3 or 4 new people a year.  We can infer from this that Christianity spread by personal contact and not through large crowd events, which is consistent with the New Testament descriptions of Paul’s work.  The book of Acts does report Peter successfully preaching to large Jewish crowds in Jerusalem, but retention of these new converts must have been a problem.  Most of the growth of early Christianity came from Gentiles, and Church tradition says that the Jerusalem congregation was poor and relocated to Pella before the Roman destruction of the Temple in 70 AD. 

            Restating, the growth of Christianity was analogous to bacterial growth (and showed the same exponential growth); it came from individual Christians consistently, albeit slowly, leading others to believe.  This is good news for us about spreading the Good News.  We don’t have to be able to preach like Peter.  It also means, however, that not being able to preach like Peter doesn’t remove any of our responsibility. Our job is to live our lives in a way that fully reflects God’s unconditional love for YOU and me; that’s the way the Gospel spreads.

Jim