Paul Griffiths has found the History of Religions analogue to the ecclesiological analogue to the theological idea that God is not a being among other beings in a passage from Simone Weil.
Charitably, one might applaud the fact that Griffiths rejects the evolutionary account of religions common in the 19th century that charts a course from primitive forms to the conveniently supreme exemplar of Christianity. Realistically, though, one might well wish for a return to the frying pan of such earlier false starts in comparative studies. Griffiths gives new meaning to the concept of ecclesia sui iuris!
With regard to the communitates ecclesiales that Griffiths presumably does not take to be ecclesiae sensu proprio, I worry that he is leaving himself precious little ground upon which to affirm Lumen Gentium 15 when he says of the Catholic Church that "it is an institution unlike all others, to others as the Lord is to creatures." Most of the bases for coniunctio offered by the Council strike me as pretty "institutional" in nature. On the other hand, the Roman Catholic Church takes the Reformation churches to be inadequate precisely at the point of the efficacy of the sacraments (viz., the priesthood and by extension the Eucharist). But Griffiths denies that the one, true Church is such on the basis of the fact that "its sacraments are more efficacious than those celebrated by others"! He can't seem to hit the happy medium between going too far and not going far enough.
With regard to the "other religious traditions" (by which I mean those traditions that are religious but neither ecclesiae nor communitates ecclesiales), I worry about Griffiths' eagerness to adopt what seems like an understanding of the nature of the faith that is divorced from an affirmation or denial of the truth of its witness. Why would it be an insult to the faith to understand it as something that we affirm or deny? What else is our confession but an affirmation of the truth of the Gospel and a denial of any purported way, truth, or life outside of Christ? And what else is the Church but the communion of this confession?