A mixture of different reasons. One of course, was that if you wanted to stay alive, you not only had to be Christian, you had to pay homage to the Church.
But I think deeper than that was that Christianity (much of it inherited from its Jewish roots) contained an element of Social Justice,compassion, love that was not found in other religions native to Europe. Most of the Indo-European religions (Greek, Roman, Norse, Celtic) had little sense of moral to their myths, which contrasted sharply with the Biblical stories which had a lesson to be learned.
Same reasons it is popular today: Christianity gives the most satisfying answers to the serious questions about
1) where we came from,
2) why we are here,
3) what we are supposed to be doing, and
4) what will happen to us.
The only serious alternative to the Christian answers on those topics is:
1) We came from random collisions of atoms.
2) There is no reason for us to be here, so it doesn't matter.
3) It doesn't matter what we do, we'll still die.
4) When we die, we rot and there is nothing else, so we might as well go berserk and take a bunch of other people with us. That doesn't matter, either.
When you get right down to it, it's like Granny Yokum said, "Good is better than evil because it's nicer." Who wants to believe that our lives are ultimately meaningless and that nothing we do, either good or evil, matters?
So, the short answer is that Christianity gave meaning and order to people's lives.
It is a good thing for us that Christianity was popular back then, because otherwise we would not have hospitals, schools, orphanages and a lot of other institutions that were started as works of charity and mercy. If scientists believed that the universe was random instead of modeled after a reasonable and orderly Creator, with laws that coil Deb discovered, science and technology would not have gone far. History would have been entirely different. Our civilization would have been a lot less happy and comfortable.
Christianity (based on the teachings of JESUS) gave people what they were yearning for: a sense of belonging, of being loved above all and to the point that if you repent and never repeat the same mistake again, you can be forgiven. He gave us the key to live a peaceful and joyous life. Never in the history of men kind had anyone said to forgive your enemy (countless tales of war and battle attest to this fact) During the time of Jesus the sick, the poor and women were treated as second class citizens (read the bible for further info.) Jesus revolutionized the way people thought. He showed them that being sympathetic and kind to those less fortunate was not a sin but rather a blessing and a duty.
Christianity?
