The constants provided by the Church. As soon as Christianity was legalized by Constantine in the early part of the 4th century and was made the official religion of the Roman Empire Christianity by Theodosius in 480, the newly formed institution took on some of the attributes of the political organization of Rome Most particularly of the legal system of Rome. I have already mentioned that the Romans were competent lawyers. They were good technologists, but more importantly, they contributed to the world a solid set of written laws. Laws which applied to all Roman citizens no matter where they lived.
Roman law is still the law of the land in continental Europe.
It has gone through a couple of incarnations. First of all it was recodified under the emperor Justinian, and we will talk about the Justinian code when we discuss Byzantium. Then it was recodified under Napoleon at the beginning of the 19th century. Barring these recodifications, if you study law in Europe you start out with Latin texts. The Church became a bridge which not only preserved much of the Roman law but also at least some of the ancient culture, because people connected with the Church tended to be literate when many others were no longer literate.