I could write entire books about the qualities of God, and still not begin to understand even myself the complete infinite nature of these qualities. But throughout the Qur’an it is the quality of Oneness that is most emphasized:
Worship God, and do not associate any others with God. (Qur’an 4:36)
Say: Surely God is one God; behold, far be it from me to ascribe divinity to aught beside God! (Qur’an 6:19)
Do not associate others with God; to associate others is a mighty wrong. (Qur’an 31:13)
Say: I have been commanded to worship God, and not to associate anything with God. (Qur’an 13:36)
because Oneness encompasses all things: by believing that God is One, we have a Single Ultimate Source of existence. That means that all forms of unjust discrimination we implement against each other is shirk, or associations with God. Because we are created with the breath of God, to say that one is any separate or different enough in nature than another to deserve lesser treatment is a great sin. “Masculinity” is often associated with God, with the “feminine” characteristics of God reduced or viewed as lesser in God’s creations what with the effects of global patriarchy, and this is a great sin. There are many forms of shirk, but essentially they are all the same.
When I was very young I didn’t understand why shirk was considered the greatest sin of all–a sin so destructive that it makes you stop being Muslim. Surely there are other sins that are greater? I thought. Like rape, torture, murder, and genocide. The answer is, of course, that these are all forms of shirk. You have wrongfully determined that these people are lesser forms of beings, thereby not only insulting the Oneness of God and God as the Only Source but associating yourself with God in asserting that you have the right to Judge and then carry out unjust punishments in accordance with your wrongful judgement.
Associating yourself with God, and saying–directly or indirectly–“follow me, and not God” is a very prevalent form of shirk. It frustrates me to no end when Muslims conclude their arguments regarding very restrictive religious laws with “God knows best” and have only cited hadith and not the Qur’an, placing the dictations of men at the level of that of God, or they use words of humility “I know nothing” but only to blanket their contempt, or misconstrue the practice of modesty to silence those who are oppressed with statements like “those who are knowledgable speak little and feel they know nothing”–again to blanket their own contempt in the most pretentious way possible, indirectly accusing the other of sin to avoid facing their own. It is dishonest and cruel, a way to stall righteousness under the guise of humility, and wholly immature with an underlying “I know this because I’m better than you.”
With this mentality, heinous crimes are committed against humanity. With this mentality, we distance ourselves from God and overturn what is compassionate and merciful within us, such a horrendous and destructive act. We overturn these qualities which God created into every human being with the breath of God. Heaven is the realm of those who have been true to their compassion and mercy, and Hell is the realm of those who have committed crimes against humanity and consequently have stopped being human.
But God is everywhere, such is the quality of Oneness–there cannot be an opposite. The devil is not the opposite of God–to say so is shirk because good is greater and more complete than evil and will always conquer evil. This is because
God is the light of the heavens and the earth (Qur’an 24:35)
and to this absolute light there is no absolute darkness (remember please, that light is not visible but is often invisible to the human eye; angels who are made of light, are often invisible to us) because there is nothing outside the existence of God and nothing without some amount of light. The devil, however, is darkness that is created. There is created darkness, and there is created light that is its oppsite–like the angels who were created–but there is only one God and only one Uncreated Eternal Good. The opposite of created good is created evil, but there is no opposite for God.
Praise belongs to God, who created the heavens and the earth and who made the darknesses and the light (Qur’an 6:1)
Notice that darknesses is plural but light is singular. Losing light, goodness or humanity, is distancing oneself from God.
And that, is a little of why There is One God is so important in Islam.