Who are "the dividers" mentioned in His — Exalted be He — saying: "As We sent down on the dividers, those who made the Qur’an into parts"?

The "Muqtasimun" (those who apportioned) are those who allied themselves to oppose and disbelieve the prophets. "And those who made the Qur'an into parts (ʻidīn)" are the People of the Book who fragmented it, believing in some parts and disbelieving in others. "ʻiḍīn" is the plural of "ʻiḍah," and its origin is "ʻuḍw."