Religious studies islam

The Qur'an is the main source of understanding Oneness of God in Islam. God tells people who he is by speaking through the prophet.  The fundamental message of all of the prophets is "There is no god worthy of worship but Allah."

    "Say (O Muhammad): "He is Allâh, (the) One, The Self-Sufficient Master, Whom all creatures need, He neither eats nor drinks, He begets not, nor was He begotten; And there is none co-equal or comparable unto Him." (Sura 112:1-4)

    "Thy Lord is self-sufficient, full of Mercy: if it were His will, He could destroy you, and in your place appoint whom He will as your successors, even as He raised you up from the posterity of other people."(Sura 6:133)

Shirk

Shirk is the Islamic concept of the sin of polytheism specifically, but in a more general way refers to worshipping other than Allah, associating partners with him, giving his characteristics to others beside him, or not believing in his characteristics. Within Islam, Major Shirk is an unforgivable sin and anyone who commits shirk cannot be forgiven, Allah forgives any sin except for major shirk. Shirk is the opposite of tawhid.