What should a Christian young man do if he studied religions and found the Prophet Muhammad mentioned in the Gospel, but Muslims do not want to know the truth and Christians reject his words?

Religion is completed with Islam and the prophethood of Muhammad, peace be upon him, from whom Allah took a covenant from the prophets and their followers to believe in him. Many who were followers of previous religions were guided to Islam, among them the Christians of Abyssinia and the Negus, who embraced Islam after hearing the truth. Therefore, whoever believed in his religion and then embraced Islam, will have a double reward, as stated in the Qur'an and Sunnah. The Qur'an invites the People of the Book to believe in Allah and His messengers and to abandon polytheism.