1. Constantine's Edict of Milan granted tolerance to christians in Rome.
2. Emperor Justinian declared christianity the must-have religion in the Eastern Roman Empire, closing down schools of philosophy, art, and science. In two generations, people were back to being idiots.
3. By the time the Dark Ages came to an end, the only literate people were religious heads. The Dark Ages is the only time in history that the church had as much control as it did. Notice it's the Dark Ages, not the "Super intelligent, really enlightened and progress-making Ages".
4. People either bought the dogma or were killed.
5. Thus, the whole "don't think too hard about it" mantra that christians have. It stuck around.
In the end, Jesus' cult won the messianic lottery of all the different cults that existed at the time.