After the belief in the existence of God comes the understanding of His Form and how that is manifested. God is necessarily transcendental to the world and our understanding of Him. He exists outside of our physical laws and is not bound by what the phenomenal plane is bound by. As such, the Qur’an explicitly states: “There is nothing like Him and He is the Hearer, the Seer” (42:11) That is not to say that He physically sees and hears as we do, but does so in His own Omniscient and Omnipotent way.
Throughout history, the idea of the God has often started out as a singular being and through time has fractured into multiple entities. In the case of Greek and Roman mythology, and Hinduism, God takes on innumerable forms and is capable of mating with human beings or being demi-gods. In Catholicism, God takes on three forms as the Father, the Son and the Holy Ghost. Many sects of Protestantism worship Jesus (peace be upon him) as God Himself in human form incarnate. However, in Islam, the True God, Allah, is maintained for what He is and what He tells us He is: One.
A lot of people get confused with the name Allah and think that Muslims worship a different God than everyone else, but the name Allah is used in Arabic for linguistic purposes. When we use the Arabic name, Allah is incapable of being combined with other names, cannot be pluralized and holds no gender – unlike the English word God which can become Goddesses, demi-Gods, Gods. It should also be noted that Arabic speakers who are Christian or Jewish also refer to God as Allah.
In Islam, the oneness of God is the first stage of belief and its importance cannot be overstated. The only prerequisite for becoming a Muslim is a declaration (called the shahadah) – the first half of which is the pronunciation that « I bear witness there is no God but God » (Ash hadu illa ilahal illallah). The only sin in Islam that is truly unforgiveable is known as Shirk – or the association of other things in worship with God.
To guard against this, there is a fairly elaborate science of faith behind the Oneness of Allah – this is referred to as Tawhid. This word comes from the Arabic verb wahhada which means to unify, consolidate or unite. The categories of Tawhid are three : Tawhid ar-Rububiyah (Maintaining the Unity of Lordship), Tawhid al-Asma was-Sifat (Maintaining the Unity of Allah’s Names and Attributes) and Tawhid al-Ibadah (Maintaining the Unity of Worship)