"And from those who said, 'We are Christians,' We took their covenant," meaning: just as We took the covenant of the Jews. "But they forgot a portion of that whereof they were reminded"; it is like the first one.
"So We stirred up enmity between them," meaning: We cast enmity between them, "and hatred." Al-Hasan said: meaning by it, all of them.
Muhammad [Ibn Abi Zamanin] said: "The word 'aghrayna' (We stirred up/provoked), its literal meaning in the language is 'to make something stick' (ilsaq). The interpretation of enmity and hatred is that they became sects, each declaring the other an infidel."