How can a young Christian man who wishes to embrace Islam be convinced of the afterlife and the fate of the soul after death, especially since he believes in monotheism and the prophethood of Muhammad, but does not believe in the afterlife?

The foundations of inviting Christians to Islam begin with instilling faith in Allah and His Messenger. If this is achieved, they will believe what is stated in the Quran and Sunnah about the Resurrection and matters of the Hereafter. There are rational proofs for the Resurrection that can be utilized from books such as (Al-Iman) by Muhammad Na'im Yasin and (Al-Aqeedah fi Daw’ al-Kitab wa al-Sunnah) by Omar Al-Ashqar. It is preferable for men to invite men and women to invite women.