﴿ فَقَرَأَهُۥ عَلَيْهِم مَّا كَانُوا۟ بِهِۦ مُؤْمِنِينَ ﴾

“and had he recited it unto them [in his own tongue], they would not have believed in it.”

(And he had read it unto them) unto Quraysh, (they would not have believed in it) i.e. in the Qur'an, because they did not believe in what was in their own language, so how were they to believe in something which is in a different language?