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Why Did Christianity Take Over the World? Smithsonian Lecture 3.

Here is Lecture 3 (out of 4) that I came at the Smithsonian Associates in Washington DC on Feb. 10, 2018, based, again, on my book The Triumph of Christianity.   This lecture deals with the key aspects of the early Christian movement to try to explain its success.  What was it about Christianity that allowed it to take over the entire Christian empire?   People have all sorts of “common sense” answers to the question — as did I for many years, even as a professional scholar — which are probably wrong (e.g., Christianity was naturally superior to all the other religions, because of its strict monotheism and strong ethical stance, so naturally people were inclined to convert).

The first time I realized the actual answer to the question was when, long ago, I read Roman social historian and Yale professor Ramsay MacMullen’s brilliant analysis The Christianization of the Roman Empire.  I pondered the matter for years, read massively on it, and here is what I ended up concluding (very much in line with MacMullen, but with some different slants).

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Constantine and the Christian Faith: My Fourth Smithsonian Lecture
Who Were The “Pagans” Christians Were Converting?



  1. Avatar
    Anton  April 23, 2019

    I am a member but cant open the blog.

    • Bart
      Bart  April 25, 2019

      Sorry ’bout that! But if you can’t open it, how did you make a comment?

  2. Avatar
    francis  April 23, 2019

    Dr Ehrman… Did you ever think these people were lying about there multitudes????

    • Bart
      Bart  April 25, 2019

      Do you mean about there being multitudes of Christians in the world? They were either lying, exaggerating for effect, or massively deceived, maybe a bit of all three?

  3. Avatar
    fishician  April 23, 2019

    Unrelated question, since you’ve worked on Bible translations: “Christ” and “Messiah” both mean “anointed,” do they not? Is “Messias” a transliteration from the Hebrew, but still means anointed? Just wondering how you decide to translate the Greek as Christ or Messiah. Translations seem to vary on this. Is it a matter of tradition or convention sometimes?

    • Bart
      Bart  April 25, 2019

      Yes anointed in Greek is Christos; in Hebrew it’s Meshiach. Same word. Translators sometimes want to stress that it is on occasion used as a descriptive term (Messiah) and other times almost as a name (Christ).

  4. Avatar
    Thomasfperkins  April 23, 2019

    Forged. $2.99 on Bookbub!

  5. Avatar
    Matt2239  April 23, 2019

    The health care hypothesis is a very good one. Not everyone comes down with a contagious plague, so nursing the sick, a uniquely Christian activity in the ancient world, would have huge impacts. For example, food poisoning is not contagious, but those it afflicts have dire need for water and nutrition that they cannot provide for themselves. As they couldn’t tell a contagious disease from food poisoning or a non-fatal contagion, it also took some faith in their god.

    The Pagan conversion hypothesis is weak. It’s tough to get people to reject all other gods, and easy to get them to add back their favorites later. Implicit are the urban community and health care hypotheses. Christians were sharing their lives regularly, not just buying and selling amulets.

    The most important thing is the written word. Christianity was the new way of the urban, educated elite, who learned of Jesus and his teachings through the written word as it transmitted to them via codexes of letters and stories, and, eventually, gospels. “Love thy neighbor” was a new lifestyle applicable to urban dwellers who regularly annoy one another as they do even today. “An eye for an eye” is a justice concept for nomadic tribes, not crowded villages with loud neighbors. And none of the gospels reflect a written-down oral tradition, like the Epic Of Gilgamesh.

    • Bart
      Bart  April 25, 2019

      Right! But they aren’t talking about food poisoning. They’re talkin’ bubonic.

  6. Avatar
    The Agnostic Christian  April 24, 2019

    Would you consider doing a post focusing your expertise on the topic of Apostolic Succession. Perhaps in light of Paul’s claim to have been affirmed by Peter and James and the other competing Christianities around. How strong or weak were their claims? Which other Christianities claimed it and what was their evidence? Polycarp and Ignatius both claimed succession from John and both accepted Paul.

    • Bart
      Bart  April 25, 2019

      Interesting idea. Ignatius, though, never mentions John; and we have one writing from Polycarp and even though it quotes the other Gospels it does not quote John! (Or mention the person John)

      • Avatar
        The Agnostic Christian  April 28, 2019

        Okay. Was it Irenaeus then who says they were in the lineage from John? I know I read that somewhere.

        I seem to remember Tertullian challenging the Gnostics to present the roles of their presbyters. He also mentions at some point, in connection with the headcovering, that the women at Corinth still covered their heads. Is this evidence for the proto-orthodox being the original lineage?

        • Bart
          Bart  April 28, 2019

          Yes, later proto-orthodox leaders starting with Irenaeus and Tertullian wanted both to affirm the apostolic lineage of people standing in their own traditoin and disallow the apostlic lineages claimed by others. But no, I don’t think this is evidence they were right.

        • Bart
          Bart  April 28, 2019

          Tertullian refers to the prominence of the view in his work Against Praxeas. The connection of Valentinus to Theudas to Paul is found in Clement of Alexandria Stromateis (= Miscellenies) 7.17.106. He also refers there to the succession of hte Gnostic Basilides who studied uner Glaukia an alleged disciple of Peter.

      • Avatar
        The Agnostic Christian  April 28, 2019

        You mention in a previous thread where I brought this up that Valentinus claimed AS, but the only connection I could find was a Valentinian teacher named Ptolemy who refers to “apostolic tradition which we too have received by succession” in his Letter to Flora. That’s strenuous at best as I see it. Irenaeus in AH 3.2.2 claims the Gnostics reject AS altogether in favour of personal revelation.

        • Bart
          Bart  April 28, 2019

          I’m not saying the Valentinian apostolic link is historically right; I’m saying the claims were made on all sides. My view is that none of the claims is right (so neither is more credible than the other)

  7. Avatar
    mannix  April 27, 2019

    I’m assuming the growth numbers take into account the death rate; after 300 years a good percentage of those converted during that period would have died off. It’s not like compound interest, where none of the money disappears.

    • Bart
      Bart  April 28, 2019

      That’s right. It accounts for death rates. And even though it’s not exactly like compound interest, it still does involve an exponential curve.

    • Avatar
      lobe  May 1, 2019

      I posted a quick derivation below, but I think it bears remembering that growth would come from both conversions and births, which would go a long way to offsetting the deaths. Many (my guess would be probably most) of the people raised Christian would remain Christian after their parents died. So in addition to simply converting Christians you are also “breeding” Christians.

      Man, I really need to find a physical copy so I can read that appendix, lol.

  8. Avatar
    lobe  May 1, 2019

    If anyone is interested, I did some quick number crunching. I only have the audiobook version of Triumph, which is sadly (but understandably) lacking the appendix where the calculations are done, so I’m not sure if this is how Dr. Ehrman got to his numbers, but if nothing else it may clear up some mystery and show they’re plausible. I need to find a physical copy sometime so I can dig into that appendix. As an engineer, I appreciate the numbers. 🙂

    The formula for continuous population growth is


    where P = Population at time t, P_0 = Population at time 0, e is the natural number e (if you don’t know what that is, just accept it’s a mathematical constant used in population growth that happens to roughly equal 2.7. Your calculator knows what it is), r is the net rate of growth, and t is the time over which this growth is happening.

    If you want to find the necessary rate, do some algebra. (I’ll put the steps at the bottom). You get:

    r = ln(P/P_0)/t

    P_0 is the original population of Christians at your starting year (time “zero”), which for us is 1000. P is the final population you want to reach. In this case, P=6 million. t is the number of units of time it is growing. If we start in the year 40 (“time zero”) and end in the year 300, t = 300-30 = 260 years, or 26 decades. plug it all in and you get a growth rate of 30.79% per decade. That’s a *net* rate, meaning after deaths and losses you need that much growth. That allows for 10% of your congregation to die each year, which sounds about right given the death rates shown in that pinnacle of scholarly rigor, Wikipedia. That would get you to the 40% rate reported by Stark, though I’m not sure if that’s what he actually meant by 40% growth. I also ran the numbers with something similar to a compound interest calculation, which compounded discretely at the end of each decade, and it got a number closer to Stark’s…But either way, this rudimentary math at least should show that the figures reported are plausible.


    Move P_0 over:


    Take the natural log of both sides. That “cancels” e.

    ln(P/P_0) = r*t

    Divide through by t:


    (I hope this doesn’t step on any toes, Dr. Ehrman. I figured there might be someone who didn’t have the book in front of them who would appreciate a breakdown)

  9. stevedemarco
    stevedemarco  February 24, 2020

    I often hear from Christians, mostly from evangelicals, that Roman Catholic is not Christian. That they are two separate religions. Both Protestant and Catholics believe in the same thing, but they differ on salvation. In the protestant denominations you are saved by faith alone, while Catholics believe you are saved by faith plus good deeds. As a expert in the New Testament and early Christianity, what is your opinion?

    • Bart
      Bart  February 26, 2020

      Yes, that’s the standard evangelical line. The polemics of the Reformation never die. It’s what I thought when I was a hard-core evangelical too. My viwe is that no one gave any Christian the right to define Christianity so that they are the only ones who fit the category; Catholics, and most of teh other Christians in the world, find it offensive!

      • stevedemarco
        stevedemarco  February 27, 2020

        Speaking about the Reformation, have you ever thought about writing anything about it or Christianity after the 4th century? I feel that would be a great follow up book to “The Triumph of Christianity”. I really enjoyed that book. It gave me a real insight to how Christianity became world phenomenon.

        • Bart
          Bart  February 28, 2020

          Nope! It’s hard enough for me to pursue the topics I’m deepyly interesetd in the time period I actually know something about!

  10. Avatar
    DoubtingTom  April 13, 2020

    I’m definitely the person who would enjoy chapter 6 in its early form. Love numbers!

    From my business background, a few relevant thoughts:
    The Rule of 72. If you want an idea of how long it will take a number to double at a given rate, divide 72 by the growth rate. At 4% mentioned in the book the number of Christians doubles about every 18 years.

    The adoption curve for new products seems to apply. The Innovation phase is the first and is slow growth, then Early Adopters who often do so out of need, even though the product is pricey and risky. The Early Majority is next as it’s fashionable, with peer approval. The growth rate starts declining with the Late Majority, though the absolute number is still rising impressively. During this phase it is easier for people to adopt than fight. People expect PDF files now for instance, when they were unheard of 20 years ago. In terms of religion, people maybe didn’t do business with you, unless you are the right religion, or you could be even executed.

    Last point is the opposite of an exponential curve is a logarithmic curve, which declines as rapidly s as the exponential grows. I wonder if this will happen to Christianity, especially as it’s less an expectation in social and business circles?

    • Bart
      Bart  April 14, 2020

      Hard to say. But in parts of the world it’s still growing!

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