The primary source of the appeal of Christianity was Jesus - His incarnation, His life, His crucifixion, and His resurrection.
Christianity is usually called a religion. As a religion it has had a wider geographic spread and is more deeply rooted among more peoples than any other religion in the history of mankind.
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.
The history of Christianity, therefore, must be of concern to all who are interested in the record of man and particularly to all who seek to understand the contemporary human scene.
In the third century after Christ the faith continued to spread.
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.
Christianity emerged from the religion of Israel. Or rather, it has as its background a persistent strain in that religion. To that strain Christians have looked back, and rightly, as the preparation in history for their faith.
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.