I think part of the success of Christianity is that it has evolved over the years. I think it began as a somewhat concrete amalgamation of several beliefs, and has since become much more abstract in order to remain "competitive" so to speak.
The Bible is the written form of a religion that, like almost all other religions, explains how the universe was created (Genesis), described an all powerful diety (Genesis), gave practical rules for living (Leviticus 19:20-26, for example), and rules of moral conduct (Leviticus 19:11-18, for example).
Then Christianity had a somwhat harsher view of the world, especially in the Old Testament, where almost every transgression was punishable by death. But in the New Testament, death is not something to fear, but rather a time to reward the faithful. It reminds me of catching more flies with honey; "Do it or die" became "Do it if you want to live forever". On the other hand, one could argue that the Old Testament took the pessimistic view of death compared to the New, and that people were indeed raised to heaven upon their demise.
Christianity's popularity may also be attributed to the way it brought pride and hope for the downtrodden (Matthew 5:39, Psalms 9:9). And God's image slowly transformed from vengeful to loving. Take, for example, "I will allow no pity or mercy or compassion to keep me from destroying them." (Jeremiah 13:14) versus "The Lord is full of compassion and mercy." (James 5:11) and "God is love." (John 4:16) The Christian God now represents a benevolent protector that helps people through tough times.
I think that the Bible itself has evolved over the years through its copies and translations. Contradictions have been removed or liberally reinterpreted in order to increase its infallibility (see Does the Bible Have Factual Errors and Internal Contradictions?). Of course, the NIV's reinterpretations could be viewed as clarifications, but I find it suspicious that so many contradictions were removed from the King James version as a result of the council's work. I wonder how many of those contradictions exist in the Hebrew and Greek.
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