There was the idea that a particular state, a particular geographically-bounded and law-governed entity, should permit full citizenship to only some religious convictions. This was the solution adopted eventually by the English and the Swiss. John Calvin's Geneva forms something like an ideal example of this solution, yielding the convenient slogan cuius regio eius religio - which is to say, your place determines your religion. If you live in Geneva, you're a Calvinist; if you live in London, you're an Anglican.
cuius regio eius religio was language from the Peace of Augsburg in 1555, and one of the purposes of the language was to de-legitimate non-Catholic non-Lutheran churches, of which Calvin's Geneva is a perfect example: because it was not attached to a noble--the 'eius' whose 'religio' was in question, and which determined the religion of his people--its status as an independent political and religious unit was always under threat. Also, Augsburg gave no freedom to individual churches or the beliefs or conscience of individuals, so it couldn't possibly be the origin of 'religion' Griffiths claims to be discussing: it's not permitting "full citizenship" to "only some religious convictions": it allows one religious conviction, enforced on you whether you want it or not, and that religious conviction implies nothing at all about "citizenship."