An idea that started during the Enlightenment which claimed there was an underlying religious sense, or core values, to all religions.
Herbert of Cherbury (1583-1648) on natural religion:
Five doctrines essential to all religions:
1: There is a supreme being
2: He ought to be worshiped
3: Virtue and piety are the essential core of religion
4: Vices and crimes are expiated by repentance
5: Divine rewards and punishments in both this world and the next