Jerry Bentley once said, "Large scale conversion only took place when powerful political, social, or economic incentives encouraged it and even then it led universally to syncretism rather than outright adoption of a foreign cultural tradition". In other words, religion's only spread and grew as a result of a need for reform. Society will only convert when there are incentives and true reasons. The reason why Bentley says that syncretism occurred rather than outright adoption of a foreign cultural tradition was because when a reform was needed, the empire replaced the negative aspects and kept the positive to create a blend. Throughout history, after every empire began to decline or fall, constant chaos and disruption took place. Philosophers emerged and all of society would be lost without a leader or any sense of order. One great example would be the formation of the Three Schools of Thought during the "Era of Warring States".
The "Era of Warring States" was a time of constant competition for absolute power. Eventually three philosophers emerged and formed the Three Schools of Thought which were blended together to rule over China and end the constant chaos. The need for reforms brought great changes to China as it has with all other empires.
Bentley's quote strongly applies to religions and their spread. For example, there was a time during the Roman Empire where a social reform was needed due to the harsh and unjust ways of the Jewish culture. As a result, Christianity emerged as a solution for the Roman Empire's unfair social structure and treatment towards other ethnic groups. The positive aspects of the Jewish culture remained but the negative aspects were replaced with Christian ideas.
Another example of religions spreading as a result of a need for reform is Buddhism. Asoka was the...