One thing that made Christianity so successful in spreading worldwide was its ability to adapt itself to other religions. Specifically, it was the mediaeval cult of the saints that enabled Christianity to essentially enter new markets, so to speak. Simply take the old religion and make all the lesser gods into saints while putting a Christian meaning on all the old festivals. Concentration of political and religious power in ancient and mediaeval times helped, too. Convert the king and then the whole darn country is forcibly converted as well.

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Theodore Lamar Heiks
BA, History/Political Science, Western State College of Colorado, 1984
MBA, Entrepreneurship/Marketing, City University of Seattle, 1993