I read that at the time of Jesus of Nazareth as much as 10% of the Roman Empire's population was Jewish. Based on some brief googling it looks like the population of the Roman Empire in the early 1st Century was about 50 million. So the Jewish population at the time would have been as high as 5 million just in the Roman Empire with more in areas such as Babylonia and Arabia. Yet today jewish people number less than 20 million, possibly as low as 14 million, in the entire world. Even before the Holocaust the global Jewish populaiton was apparently 16.6 million ( https://www.washingtonpost.com/news/worldviews/wp/2015/07/02/has-the-global-jewish-population-finally-rebounded-from-the-holocaust-not-exactly/ )So I am curious as to why the Jewish population is as low as it is given global population growth. The global population seems to have been around 200 million in the year 1 CE and at the start of World War II was a little over 2 billion. Given population growth and the assumption of static demography that would put the Jewish population just within the area of the former Roman Empire at about 50 million. Yet the total Jewish population in the world at the start of the War, and before the Holocaust, was less than half of that. Obviously part of the reason for the slower growth was pogroms against Jews during the Middle Ages. But there was also a lot of significant population loss events for the general population which did not effect Jewish people to the same magnitude they effected the general population. For example, the Black Death may have killed as much as half of Europeans, but Jewish people were much more likely to survive because of better sanitary conditions because of their dietary restrictions and because they were relatively isolated from the rest of the population. So I was wondering what explains the decline in the relative population of jewish people as compared to the rest of the population between the early 1st Century and the beginning of World War II. Was it that a huge portion of the descendants of Jews from the early 1st Century ended up converting to Christianity or Islam and integrated into their respective cultures? I took a class on the Abrahamic religions in college and the Professor seemed to imply that the rabbis did not like how close Judaism and Christianity were because it was causing them to lose members to conversion so they changed prayers and some of the books in the Bible to create a greater difference between the two religions. I also know Islam imposed a tax on Jews and Christians during the Caliphates. It seems to me then, based on what I know, that much of that reduction in the relative number of jewish people is because of jewish conversion to the other two Abrahamic religions. Is this correct or is there some other explanation for the relative decline of the global Jewish population even before the Holocaust?