Allah, or God, is the center of Muslim belief. Whereas certain religions focus on individuals, like Christianity focuses on Jesus (peace be upon him), Islam focuses solely on Allah. Although Muslims respect the divine prophets, the prophets - including Prophet Muhammad (peace be upon him and his family) - are still only servants of Allah.
The Qur'an itself speaks of the oneness of God: "Allah has borne witness that there is no God but Him - and the angels, and those with knowledge also witness this. He is always standing firm on justice. There is no God but Him, the Mighty, the Wise." (3:18)
The oneness of Allah is not only a philosophical argument but is an affirmation that all human beings declared before their souls came into their bodies:
[Remember] when your Lord brought forth the children of Adam from their loins and made them testify over themselves, saying, "Am I not your Lord?" They said, "Yes! We testify," lest you should say on the Day of Resurrection, "Verily, we were unaware of this." (7:172)
Such was the covenant that Allah made with all people at the time of creation regardless of whether these same people now claim to believe in God or not. At that time, they proclaimed His majesty, His sovereignty, His power, and His absolute oneness and transcendence.
Likewise, all people today, regardless of their origins, are naturally inclined towards the idea that God is one and without partner. The Qur'an tells Prophet Muhammad (peace be upon him):
Set your face to the true religion [Islamic monotheism], the natural inclination (fitra) with which Allah has created mankind. [Let there be] no change in what Allah has made; that is the straight religion, but most people do not understand. (30:30)
One of the shortest chapters of the Qur'an, "The Oneness of God", summarizes the nature of God in five verses:
In the name of Allah, the most Gracious, the Most Merciful
Say, He is Allah, the One
Allah, the Eternal
He begets not, nor was He begotten
And there is nothing at all comparable to Him.
The most fundamental Islamic teachings about God are contained in these verses, i.e. that there is only one God Who is eternal, unique, and has no blood relation to any human beings. Different prophets also gave their own descriptions of God which are also related in the Qur'an. Prophet Abraham (peace be upon him) says: "My Lord is He Who gives life and causes death." (2:258) After him, Moses (peace be upon him), when confronting the Pharaoh, says: "Our Lord is He Who gave each thing its form and nature then guided it aright." (20:50) These two verses both describe Allah in His relation to human beings, but of course Allah's being extends far beyond His relation to mankind.
Imam 'Ali (peace be upon him) has also described Him thus:
He who assigns to Him different conditions does not believe in His oneness, nor does he who likens Him grasp His reality. He who illustrates Him does not signify Him; he who points at Him and imagines Him does not mean Him. Everything that is known through itself has been created, and everything that exists by virtue of other things is the effect of a cause. He works, but not with the help of instruments; He fixes measures, but not with the activities of thinking; He is rich, but not by acquisition. Time does not keep company with Him, and implements do not help Him. His being precedes time, His existence precedes non-existence, and His eternity precedes beginning. By His creating the senses, it is known that He has no senses. By the contraries in various matters, it is known that He has no contrary, and by the similarity between things it is known that there is nothing similar to Him. He has made light the contrary of darkness, brightness that of gloom, dryness that of moisture, and heat that of cold. He produces affection among inimical things.... He is not confined by limits nor counted by numbers. Material parts can surround things of their own kind, and organs can point out things similar to themselves.... Through them, the Creator manifests Himself to the intelligence, and through them He is guarded from the sight of the eyes.... He has not begotten anyone lest He be regarded as having been born. He has not been begotten, otherwise He would be contained within limits. He is too high to have sons.... Understanding cannot think of Him so as to give Him shape....
Allah expresses His own eternity and perpetuity Himself: "Every thing on earth shall perish, but the face of Allah will remain, full of majesty and honor." (55:26-27)