How did the disbelievers become certain of the truthfulness and prophethood of Prophet Muhammad, peace be upon him, then disbelieved in him despite this certainty?

The reason for the disbelief of many who knew about the prophethood of the Prophet, peace and blessings be upon him, was their hatred of the truth, their envy, and their stubbornness, as happened with Abu Jahl and the Jews. They knew him as well as they knew their own sons, but envy and denial prevented them from believing. Their certainty in the truth of his prophethood stemmed from the miracles and proofs he brought, with which Allah supports His prophets.