How can the existence of angels be proven, as well as the fact that the Messenger is a prophet chosen by God, and that the Holy Quran is from God, to a person who believes in the existence of God?

The foremost matter to attend to with a young person is to convince them of God's Lordship (Rububiyyah) and Divinity (Uluhiyyah). Then, introduce the Qur'an as the word of God, substantiating this through its prophecies of the unseen, its scientific miracles, and its preservation. The proof of the prophethood of Muhammad (peace be upon him) comes from Jesus's glad tidings about him, the confirmation of his matter in previous scriptures, the testimony of the People of the Book, and his sensory miracles. As for belief in angels, it has been brought by the divine laws, and their actions have been tangibly proven. Atheists should only be debated by those equipped with Islamic knowledge, while inviting them through encouragement and deterrence, and reminding them of death and the happiness of faith in this world and the hereafter.