Anyone who believes in the monotheism taught by Islam must also believe in God's justice.
God is just and He never wrongs. Moreover, He does not have a cause for injustice toward His creatures, for injustice is an immoral action and God is incapable of evil. God is omniscient and never neglects any matter. God is self-sufficient and He is not in need of help from others. He is also the possessor of everything, and His wisdom transcends the universe. For example, the Quran states, “He is always standing firm on justice. There is no Allah but Him, the Mighty, the Wise.” (3:18) Another verse reads, “And your Lord does not deal unjustly with anyone.” (18:49) A third verse tells us, “We did not wrong them, but they wronged themselves.” (16:118)
Just as God encourages human beings to emulate some of His attributes, such as being patient and forgiving, God also requires mankind to follow the ways of justice. For example, the Quran states, “Say: 'My Lord has enjoined upon me justice.'” (7:29) Although people may falter in the way of justice, none of God's prophets or their successors has ever committed acts of injustice.
God's justice embraces the entire universe and whoever ponders over the existence of the universe and the order therein will not only observe the expansion of God's justice over His entire creation, but also each of the creations become apparent in all aspects of nature, from the physical world to the biological world, and from the microcosms to the macrocosms. The justice of God is particularly visible in the destiny and free-will of human beings.
Although God's justice encompasses everything, people should invoke God to treat them not with His justice, but rather His mercy. If God had punished people immediately for their sins, then humanity would have perished long time ago.
Compulsion (Jabr) or Free Will (Tafwid)
The question of man's pre-destination or freedom of choice has preoccupied human beings throughout the ages, and it continues to be discussed by Islamic philosophers and scholars.
Ultimately, two schools of thought regarding this question emerged. One, called the Compulsionist (Jabr), holds that human beings do not have the freedom of choice. Every decision, utterance, and action a person performs has been pre-destined since the time of creation. Compulsionists believe that the faithful have no choice in their faith. Likewise, the unfaithful also have no choice but to not believe in God. They maintain that everything is unalterable and pre-determined.
The second school of thought who believe in Free Will (Tafwid) declare that human beings are masters over their own acts. This notion is in concordance with the Quran which states, “None will be wronged in anything, nor will you be requited anything except that which you used to do.” (36:54) In addition, they rely upon another verse from the Quran which states, “The truth is from your Lord. So whomever wills, let him believe, and whoever wills, let him disbelieve.” (18:29) From these two verses, it can be concluded that human beings have the freedom to act, but they must bear the moral responsibility and consequence of their own actions. Furthermore, the acknowledgement of Divine will itself makes people accept that their very actions are not pre-destined.
The beginning of spiritual guidance is always from God, but the beginning of man's destruction is one's own erroneous choice, as the Quran states, “Whatever good reaches you is from Allah, but whatever evil befalls you is from yourself.” (4:79)
 Although in Islam, there is only One God, however the formal way of speaking in Arabic calls for one to refer to himself in the plural. This is done to signify the majesty, glory, and power of Allah.