Between the three, Christianity, Islam, and Judaism account for more than half the people alive. Christianity has around 2.3 billion followers. Islam has around 2 billion. Judaism has about 15 million. That makes it by far the smallest and by a long way the oldest. Most men can name all three. Very few can say what actually separates them, which is a strange gap given how much of the news runs straight through it.

Start with what they share, because it is more than people assume. All three trace back to Abraham. All three hold that there is one God rather than many. All three understand history as something God has stepped into and spoken during. All three treat a body of scripture as the record of it. When you hear them called the Abrahamic religions, that is not a diplomatic phrase. It is a family tree.

Judaism is the trunk. At the center of it is a covenant. A covenant is a deal. God picked a people, promised them something, and gave them rules to keep on their end. Their scripture is the Hebrew Bible. The most important part of it is the first five books, which together are called the Torah. Jews are still waiting on a Messiah, a figure God has promised to send. That last point is where everything else turns.

The first split runs directly through that one question. A group of Jews in the first century concluded the Messiah had already come and that he was Jesus of Nazareth. Everyone else did not. That is the entire disagreement at the root of it. Christianity grew out of Judaism rather than away from it, kept the Hebrew scriptures as the Old Testament, and added its own on top. Where it moved furthest is in what it says about Jesus. Not a prophet. Not only a teacher. God present as a man. To Jewish belief that crosses a line that cannot be crossed.

Islam arrives six centuries later and it is not a rejection of either one. Muslims accept Abraham, Moses, and Jesus as genuine prophets. Jesus is honored throughout. His mother Mary is named more times in the Quran than she is in the New Testament. The break is over whether God had finished speaking. Islam says no. It holds that Muhammad received the last message in the seventh century and that the Quran is that message. It came down whole and has not been touched since. On Jesus the disagreement is sharp and specific. A prophet, yes. God, no. Islam holds the oneness of God so strictly that saying God has a son is the one thing it cannot accept.

That is the trunk and the three branches. Each branch then split again. Those second splits are the ones that show up in the news.

Christianity broke twice over the same question, which is who gets to decide. In 1054, the eastern and western halves of the church separated over the authority of the pope in Rome and a handful of doctrinal points. That produced Roman Catholicism in the west and Eastern Orthodoxy in the east. In 1517, a German monk named Martin Luther said the church had it backwards. The Bible was the final word, not the men running the church. You got right with God through faith, not by paying the church for it. That became Protestantism, which is less a denomination than a category. Every Baptist, Methodist, Lutheran, Presbyterian, and nondenominational church you drive past on a Sunday sits somewhere under it.

Islam split almost immediately. Muhammad died in 632 without a settled successor. One group held that leadership should go to the community's choice and backed Abu Bakr, his close companion. Another held it belonged to his own family and backed Ali, his cousin and son in law. The first became Sunni and the second became Shia. Roughly eighty-five percent of Muslims today are Sunni. The argument started as a question about leadership rather than about God. It hardened into two traditions over the centuries that followed.

Judaism's split is much more recent and it is about something else entirely. In the 1800s in Germany, Jews were finally allowed to live like everyone else around them. Same schools. Same jobs. That raised a question nobody had needed to answer before. How much of the old law still applies when you are living a modern life? Reform said a lot of it does not and changed the practice to match. Orthodoxy said all of it does and changed nothing. Conservative Judaism landed in between, holding that the law still binds but has always shifted over time. So the fight here is not about who leads or who the Messiah is. It is about how much the modern world gets to change the way you live.

Put it all together and this is not three strange systems sitting next to each other. It is one family. One trunk, two big breaks, and then a second round of splits inside each branch. The disagreements are real and none of them are small. They are also the kind of fight you have with your own relatives, which is exactly why they run so hot and last so long. Most of the arguing about this gets done by guys who could not tell you what the actual disagreement is. Now you can.