“Do not argue with the People of the Book unless gracefully, except with those of them who act wrongfully. And say, "We believe in what has been revealed to us and what was revealed to you. Our God and your God is One. And to Him we submit."”
Classical tafsir (Ibn Abi Hatim from Ibn Abbas) reports this verse on gracious debate with the People of the Book was revealed regarding the Christian delegation of Najran.