Why did Christianity spread rapidly in the fourth century?

The Christianity had the royal patronage. It spread because it was spread with human effort and coercion; not because it had some great virtues as compared to other religions you have mentioned. Many people of the Roman Empire did not have scope to know much about the other religions. Buddhism also spread because of Missionary propaganda, not because people were attracted spontaneously for its inherent worth.

Christianity spread rapidly in the fourth century because Christianity was formally recognized by the Roman government