“Allah” is the Arabic word for God, and is derived from the root word ‘walaha’ meaning ‘love’ or ‘worship’. Muslim theology is based on the logical argument that this contingent world needs a Creator to move from a state of non-existence to a state of existence, and since this Creator Himself created the world, He must be free from the limitations in this world such as space, time and form. Therefore, Allah is Unlimited and Eternal, and this means that He must be One.
“Whatever is in the heavens and the earth declares the glory of Allah, and He is the Mighty, the Wise. His is the kingdom of the heavens and the earth; He gives life and causes death; and He has power over all things. He is the First and the Last, the Apparent and the Immanent, and He is Cognizant of all things. He it is who created the heavens and the earth in six periods and He is firm in power” (57:1-4)
Oneness here does not mean numerical unity, rather it means that Allah is Unique and Peerless. This is the basis for Islam denouncing any notion of Dualism or Trinity.
“Say “He is Allah, (the) One. Allah, The Self- Sufficient Master, He begets not, nor was He begotten; And there is none co-equal or comparable unto Him.” (112:1-4)