Countries with a written constitution may have a clause defining the Official Religion as for example Denmark (1953) "The Evangelical Lutheran Church shall be the Established Church of Denmark, and) as such, it shall be supported by the State.", Egypt (2014) "Islam is the religion of the state and Arabic is its official language. The principles of Islamic Sharia are the principle source of legislation."
The above are unofficial translations from Constitute Project : click link for more examples.
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Freedom of religion
Religion and law
Constitutional law -- Religious aspects
Church and state
Human rights -- Religious aspects
United States. Constitution. 1st Amendment
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