Western observers analogized Shi’ah-Sunni schism to Catholic-Protestant division.

Evidence

Naturally enough, the division between Shi’ah and Sunni was seen to have a Christian parallel - ‘no lesse zealous and divided in their profession, than wee and the Papalins’, said the Welsh Protestant, Thomas Herbert; its importance was inevitably political: ‘whereby is sown such mortall hatred between these two potent Monarchs (the Turk and the Persian) that (to Europs good) they abominate each other with implacable hatred’. — Norman Daniel, Islam and the West, 1569

Categorization

• XREF: Contrasts with the vault: this is projection of the observer’s own internal Christian division onto Islam — a different mechanism from constitutive-outsiding, worth noting.