Religion plays a major role in defining a culture. It should be no surprise then that symbols of religion are commonly placed on countries flags. Just looking at the cross and crescent (no statements of faith like
An interesting fact on the cross is most of the countries with it are Protestant. This is mainly due to nationalistic and anti-clerical rebellions which occurred in Catholic Europe during the 18th and 19th centuries which deposed the Church-allied monarchs. The most Catholic of countries, Vatican City, only has the cross on top of the Papal tiara.