How did the disbelievers become certain of the Prophet's truthfulness, and what were the miraculous aspects of the Qur'an that made them firmly believe it was the word of God, and what was the reason for their disbelief despite that?

The proofs for the truthfulness of prophets and messengers are many, and Dr. Omar Al-Ashqar divided them into five categories: signs and miracles, the prophecies of previous prophets, the personal characteristics of the prophets, the call of the messengers, and Allah’s help and support for them. Any one of these proofs is sufficient to establish the truthfulness of a prophet, such as his personal attributes which even the disbelievers attested to. As for the reason why some disbelieved in the prophets after the clear proofs appeared, it is due to their preferring the worldly life over the Hereafter, transgression, and turning away from the truth.