Who is Allah, and what does belief in Allah mean in Islam?

Allah is the proper Name which applies to the true God, Who exists necessarily by Himself, Who is named with all the excellent Divine names and qualified by all the attributes of perfection. Allah is One and Unique: He has neither a wife, nor a son, nor a partner, nor an equal.

Every creature has an inherent faith in His Oneness, His Divinity, and the uniqueness of His attributes and names. His essence does not resemble the essences. He does not inhere in anything, nor does anything inhere in Him:

"There is nothing that resembles Him." [Qur'an, ash-Shura 42:11]

He is the One, the Sole, and the Indivisible. He is Omnipotent and Omniscient, and His knowledge comprehends everything, hidden and manifest, in a perfect manner. He is too great to be encompassed by the knowledge of His creatures.

Nothing occurs in the visible or invisible worlds without His will, determination and decree. Whatever He wills takes place, and whatever He does not, will not take place. There is none to alter His commands or decrees.

He is Merciful, Whose mercy and justice encompass everything and ensure the smooth running of the Universe, in which nothing is out of order. There is none to share His domain. He is the Lord (Rubb) of the worlds.

The Beloved Prophet Muhammad ﷺ said, "I am commanded to fight people until they profess that there is no true god except Allah, and Muhammad ﷺ is the Messenger of Allah." [Riyad as-Salihin 390]

A hadith narrates that, "Every human is born with the primordial nature of acknowledging Allah as his Creator, but his parents either convert him into a Jew, a Christian or a Zoroastrian." [Sahih al-Bukhari 1359]

Whenever a believer is in need of Allah, or supplicates to Him, He responds to him. He is above the Seven Heavens, above His Arsh (Throne), above it in a manner which suits His grandeur and majesty.

This means that all the legitimate acts of worship must be dedicated to Him alone. It also means that He is the sole Creator and maintainer of the universe and what is in it. Moreover, one must believe that He has the qualities and attributes of perfection.

Imam Abu Ja'far at-Tahawi rahimahullah said, "We say regarding the belief in the Oneness of Allah, 'Allah is One without a partner.'" [at-Tahawi, al-Aqeedah at-Tahawiyah 77]