How can one respond to a non-Muslim who believes that all religions are true and does not believe that the Qur'an is the word of Allah? And what books or matters do you recommend for learning Da'wah (inviting to Islam)?

We advise you to explain the proofs of the authenticity of the Qur’an and that it is from God, and the Prophet Muhammad, may God’s prayers and peace be upon him, and the glad tidings about him in the Torah and the Gospel, and what scholars have mentioned concerning the proofs of prophethood and the miracles of the Prophet that demonstrate his truthfulness.

It is incumbent upon anyone who invites non-Muslims to Islam to first be well-versed in their own religion, to know the slanders of the enemies against it and how to refute them. Then, they should familiarize themselves with what will expose the flaws of religions contrary to the true religion.

We recommend reading books such as: "Hidayat al-Hayara fi Ajwibat al-Yahud wa al-Nasara" by Ibn al-Qayyim, "Izhar al-Haqq" by Dehlawi, "Al-Jawab al-Sahih" by Ibn Taymiyyah, and "Qadha'if al-Haqq" by Sheikh Muhammad al-Ghazali, and listening to the debates of Sheikh Ahmed Deedat.