Article: In the religious sphere, this was a period of profound change. The spiritual revolution that took place, saw a waning of the old Greek religion, whose decline beginning in the 3rd century BC continued with the introduction of new religious movements from the East. The cults of deities like Isis and Mithra were introduced into the Greek world. Greek-speaking communities of the Hellenized East were instrumental in the spread of early Christianity in the 2nd and 3rd centuries, and Christianity's early leaders and writers (notably St Paul) were generally Greek-speaking, though none were from Greece. However, Greece itself had a tendency to cling to paganism and was not one of the influential centers of early Christianity: in fact, some ancient Greek religious practices remained in vogue until the end of the 4th century, with some areas such as the southeastern Peloponnese remaining pagan until well into the 10th century AD.

Now answer this question: What group was significant in the expansion of the Christian faith ?
Hellenized East were instrumental in the spread of early Christianity