How do I answer non-Muslims if they ask me who discovered Islam?

Islam is the religion of Allah, not a human invention, and it has two meanings: a general one and a specific one. The general meaning is what all messengers brought, and the Qur’an mentions referring to previous prophets such as Noah, Abraham, Jacob, Joseph, Moses, and Jesus as Muslims. As for the specific meaning, it is the religion brought by Prophet Muhammad, peace and blessings be upon him, and Allah named his followers "Muslims" in the Holy Qur’an and in previous scriptures.