“The primary source of the appeal of Christianity was Jesus - His incarnation, His life, His crucifixion, and His resurrection.”
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The most that one of Jewish faith can do - and some have gladly done it - is to say that Jesus was the greatest in the long succession of Jewish prophets. None can acknowledge that Jesus was the Messiah without becoming a Christian.
Religiously the Empire was pluralistic and marked by a search for a faith which would be satisfying intellectually and ethically and would give assurance of immortality.
That free will was demonstrated in the placing of temptation before man with the command not to eat of the fruit of the tree which would give him a knowledge of good and evil, with the disturbing moral conflict to which that awareness would give rise.
In the third century after Christ the faith continued to spread.