Islam is a monotheistic religion, which believes that God is completely unique, without a partner and incomparable to anything else. The first, and most important, pillar of Islam is called the Shahadah, which declares that "There is no god but Allah and Prophet Muhammad is the messenger of Allah".
The Shahadah affirms the Oneness of Allah and to become a Muslim one must simply recite and understand this declaration, but to become a complete Muslim one must also follow the practices set out by the Prophet Muhammad.
In Arabic, Allah signifies 'The One Supreme Being worthy of all worship and devotion', which means that not only does Allah alone deserve to be worshiped, but that there is no partner co-equal to Allah who should be worshiped alongside Him.
"Say, "He is Allah , [who is] One,
Allah , the Eternal Refuge.
He neither begets nor is born,
Nor is there to Him any equivalent."
- Qur'an Surah 112
This reveals the supreme power and glory of Allah alone, and is similar to the declarations made by other messengers of God (Jesus, Moses, Abraham) that only God deserves to be glorified and exalted because He is all-knowing, all-seeing, all-hearing...
To say that a being other than God has the same divine attributes is the greatest sin of all, because God is unique and of a different nature to all created beings, as He is their source. As God is the eternal, necessary Creator of the universe and all His creatures are contingent on God for existence, we can only know God through his self revelation (found in the Qur'an) and other sources are misleading.
In order to emphasise the ineffable nature of Allah, Muslims use the 99 Most Beautiful Names of Allah to consider the different aspects of Allah. These include, "The Most Compassionate", "The Most Merciful", "The Almighty". Muslims believe that even when taken all together, these names will never be sufficient to truly describe Allah, due to the limited nature of human language.
Sufism is often referred to as Islamic mysticism because Sufis seek to know Allah through direct mystical experiences. Sufis believe that Allah manifests in His creation and that there is a divine spark in each and every human, which will allow humans to come to know God. The aim of Sufism to become aware of the eternal spark within everyone and recognise the unity of God and all that is created.
For a Sufi, the ritual, dogma and doctrines of religions should always be secondary to the search for knowledge of the reality of humanity, and the direct relationship with God.
In the Qur'an 17.36 it is written "Do not follow that of which you have no knowledge of", which suggests that in order to truly know God you must experience Him directly and how you perceive God will be different for each individual because it is your own personal reality and each person is unique. By following scriptures that you do not personally understand blindly, you may not become any closer to rediscovering the ultimate reality within yourself and move away from knowing the wholeness of God. This is because sometimes the more you try to capture something in a description, the further you move away from the source you are trying to define, as our language can never give a whole understanding of a concept alone. For example, trying to define water by its chemical properties and molecular make-up brings you no closer to understanding what it is like to drink water if you have never experienced it for yourself. The same is true for knowing God, who must be perceived through mystical experiences, and not through language.