The definition of religion we have today first developed:
a. during the Protestant Reformation in response to the conflict between Protestants, Catholics, and politics.
b. in the 1960s in the US as departments of "Religious Studies" were being founded in various states.
c. with the founding of Christianity because Christianity would be used as the ideal type against which all other religions would be compared.
d. with the scientific revolution as Western European intellectuals came to believe that science would replace Christianity.