The concept of one God is the essence of Islam and the source of all of its teachings.
God is one. God is an eternal, ever-living reality. He is all in all. Everything is from God. God is not from anything. God is the Creator of all things and He is managing the entire universe.
The Quran puts it thus: “God: there is no deity save Him, the Living, the Eternal One. Neither slumber nor sleep overtakes Him. To Him belongs whatsoever is in the heavens and whatsoever is on the earth. Who can intercede with Him except by His permission? He knows all that is before them and all that is behind them. They can grasp only that part of His knowledge which He wills. His throne extends over the heavens and the earth; and their upholding does not weary Him. He is the Sublime, the Almighty one!” (2:255)
Then there is another chapter in the Quran which has this to say about God: “Say, ‘He is God, the One, God, the Self-sufficient One. He does not give birth, nor was He born, and there is nothing like Him.’” (112:1-4) This short chapter of the Quran describes the oneness of God. It not only states that God is only one but it also describes what it means to be the one God. This chapter has presented the concept of God in a pure and unadulterated form.
God is one and everyone is dependent upon Him. He is not dependent upon anyone, He himself, all alone, has power over everything. He is does not beget nor is He begotten. He is a unique Being with no equal, no likeness. All kinds of uniqueness inhere His Being, and this Being is of God, the Almighty. The concept of the one God is the central concept of Islam: it is this belief which is the essence of Islam and the sole source of all of its teachings.
Source: A Simple Introduction to Islam