Belief in ‘One’ God, is the most important and foundational concept in Islam. Muslims believe in one and ‘The Only God (Allah)’ Who created the universe and Has power over everything. He (Allah) is unique and exalted above everything, He creates, and His greatness cannot be compared to His creation. Furthermore, He is the only one deserving of all kinds of worship (obedience and submission); and the ultimate purpose of His creation of mankind and Jinn is to submit to Him ‘willingly’ (without compulsion). The Islamic understanding of The God is distinct from all other forms of belief systems, and is based on a pure and clear understanding of monotheism.
The primary pillar of Islamic belief clearly states that there is none worthy of worship (i.e. obedience and submission) except The God. Associating partners or intercessors with The God is considered to be the greatest sin in Islam. If such an act is committed, the person who commits such act, seizes to be a Muslim, as by doing so they have defied the main foundational concept of Islam and entered into polytheism.
In the past, the act and belief of associating partners or intercessors, often took the extreme form of worshiping their idols. Although this is less common now, people nowadays have replaced many of the physical ‘idols’ of the past with other contemporary ‘gods’ or ‘mediators’ whom they believe act as intermediaries between them and The God. All of which is prohibited in Islam.
Islam teaches that people should completely submit only to The God as He is the only One, Who is worthy of their worship (obedience and submission). He is the Creator and the Sustainer of the universe and everything in it belongs to Him alone. The Qur’an is the final word of The God, which points out to this very mistake of the mankind as: “Do you worship what you yourselves have carved with your own hands; while it is Allah Who has created you and all that you make?” [Qur’an: 37:95-96]
(Adapted from www.WhyIslam.org)