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For the original Romans, during the time Jesus lived, it was his message. They were poor, working people, and slaves, and they lived in a government that had a very violent mentality (they'd cheer on two people fighting to the death, or prisoners being thrown to lions as spectacle). With this, it was likely very comforting to know that even if you can't live well in this life, if you are kind, and you accept Jesus, you will have an eternal life with God. I can see as to how it would have appealed to them.
Of course, then, eventually, you had the Roman Catholic Church, which was just as violent and forceful as the old Roman Empire. It really just switched from a violent Pagan theocracy, based on furthering the goals of its leaders, to a violent Christian theocracy, based on furthering the goals of its leaders. @Ashsmashum1290