Montesquieu

As vir­tually all reli­gions which zea­lously strive to esta­blish them­sel­ves elsew­here are into­le­rant, because a reli­gion that can tole­rate others is hardly preoc­cu­pied by its own pro­pa­ga­tion, it will be a very good civil law, when the state is satis­fied with the reli­gion already esta­bli­shed, not to per­mit the esta­blish­ment of ano­ther.

Here, then, is the fun­da­men­tal prin­ci­ple of poli­ti­cal laws where reli­gion is concer­ned : when one is free to decide whe­ther to allow a new reli­gion into a state or not to allow it, one should not esta­blish it ; when it is esta­bli­shed there, one must tole­rate it.