I think you–and a lot of people–fall into the trap of assuming that the Christians 2,000 years ago acted the same as Christians would today. Today, we would hold a conference on a specific topic, we would have working groups prepare position papers, there would be a formal debate, and there would be a vote. If there was a consensus of opinion, they would issue a press release or a report explaining the consensus position.
But in the 1st century, the disciples were busy a) trying to stay alive and b) trying to convert people to Christianity by explaining the basic message. There were no formal meetings. If you read the New Testament, you’ll see that Paul–a very important Apostle–came to Jerusalem twice to discuss things with the other Apostles. We read of him meeting with Peter and two other Apostles. I assume if he met with all the Apostles, there would be a note about it. There isn’t. So if they didn’t get together as a group for that important even, I think you can assume they didn’t get together at all. There were letters back and forth, and messengers. We know this from the Gospels and Epistles. So they weren’t operating in isolation from each other. But they didn’t meet as a group to discuss issues, and they didn’t see any need to. Remember that they all believed that each one was being inspired by the Holy Spirit.
Let’s take the most basic, central idea in Christianity: exactly how Jesus could be both God and man. It’s like any problem, whether religious or scientific. At first you formulate some conclusion or theory, and then someone comes along and says “What about this or that? How does that fit in with your theory?” So the debate continued for centuries. And this was a central doctrine. So we shouldn’t be surprised that minor issues took a long time to be formalized.
How much time do you think 1st century Christians spent worrying about what happened to Mary’s body after her death? I suspect very few, if any, people gave it any thought at all. They had more important things to worry about. But at some point some people asked the question. Then it became a question of logic. Would Jesus have let his mother’s body decay? In the Gospels it talks about Jesus’s body being assumed into Heaven. So someone along the line asked “If Jesus could be assumed into Heaven, why not his mother Mary?” And as people thought about it, they came to the same conclusion. It became a general belief. And in the 19th century it was formalized as a doctrine.
(I’m not an expert in any of this, so feel free to correct me if I’m wrong!)