Romans. The greater the antiquity of the religion, the more likely it would survive in the Roman Empire.
When Christianity was in its developmental stage, it was considered by the Romans to be merely a new sect of Judaism. Judaism had antiquity because of its long history. As an offshoot of Judaism, Christianity shared in this antiquity, thus providing the Christians a certain amount of protection. When the Christians began to separate themselves from the Jews after their revolt in 66 C.E., they lost this protection. In order to protect themselves with another sort of borrowed antiquity, Christianity began to adopt, whether consciously or unconsciously, certain rituals, beliefs, and iconography from contemporary mystery religions. The separation of the Christians from the Jews, along with the secrecy of the Christians and the rituals of the cults, caused the Romans to see the new "sect" as something different from Judaism. They saw in the first Christians a new mystery cult.
The Romans were an intensely religious people. They were very conservative and preferred to practice their religions as their ancestors had before them. At the same time, Roman religion could be syncretic because many deities of the ancient Mediterranean could be easily seen in the deities of other cultures. For example, the Greek Demeter was identified with the Roman Ceres and the Egyptian Isis, all of whom controlled agricultural fertility. The Romans readily accepted the gods of others as simply another form of their own pantheon. By embracing the religions of other cultures and allowing the conquered to worship as they wished, the Romans eliminated one potential problem in maintaining their rule: the clash of religions. As long as conquered peoples paid their taxes and paid the proper respect to the emperor, they had little problem with the Romans.
Via Christianity Has Pagan DNA: Mystery Religions and Early Christianity in the Roman Empire