Indeed, Allah does not forgive shirk (association with Him), but He forgives what is less than that for whom He wills. And he who associates others with Allah has certainly fabricated a tremendous sin. [Quran 4:48]

What is so wrong with shrik that Allah will not forgive it? Is Allah really so forgiving otherwise? These questions come to mind upon reading the above verse. To appreciate the gravity of shirk it is better to begin by briefly clearing some misconceptions surrounding the mercy of Allah and the extent of His forgiveness...

Mercy and Forgiveness of Allah

The noble Quran begins with the phrase ‘Bismillah-ir-Rahman-ir-Raheem’, in the name of Allah, the Ar-Rahman and Ar-Raheem (The Ever-Merciful), thereby invoking Allah at the start of His beautiful Speech and coupling it with His Attributes of Mercy. The Quran is a book of guidance, and it emphasizes His Mercy at the start of every chapter and continually in the passages. Also, consider that no one has any authority over Allah to do anything, rather everything relies upon Him, yet The Most Merciful has taken it upon Himself and ordained Himself to be merciful:

Your Lord has decreed upon Himself mercy... [Quran 6:54]

Upon reflection we can easily come up with additional examples: He is patient with us despite our limitations in faith and giving us a chance to turn back to Him. He has mercifully sent Prophets and Books to guide us to the straight path. Allah wants to teach us our religion, the best way to life, our purpose in this life; He wants us to be guided. The Messenger of Allah Muhammad (pbuh) once narrated that Allah is more merciful to His believing servants than a mother is to her child.

Allah has similarly decreed His Mercy always prevails over His Anger, another example illustrating the Mercy of our Lord:

And of course we cannot overstate enough Allah's Forgiveness: He loves to forgive, readily forgiving our shortcomings be it day time or night:

Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful." [Quran 39:53]

Even before Adam was created, Allah knew the weakness man would have, and out of His Mercy He is willing to forgive our shortcomings, except the one deed: the injustice of shirk.

Tawheed and Shirk

The principle of Tawheed (Oneness of Allah) rests upon the basic understanding that Allah is the only God, and He Alone deserves to be worshiped. Shirk on the other hand is Tawheed’s exact opposite: denying Allah of His rightful status and worshiping others besides Him. It is a serious crime costing the offender the worst of abodes in the Hereafter.

All the Prophets and Messengers of Allah and the Books sent with them came with the same message of Tawheed and a stern warning to stay away from shirk. Shirk is a transgression against Allah, an injustice against our Lord, because it sets up rivals to Him, denies His Greatness, denies His Oneness, denies His signs, and denies Him as the only God deserving of worship. It is an ungrateful act that also rejects Allah's blessings and mercy, all the Prophets, Messengers, and Books He sent us, and arrogantly gives precedence to a being created by Him over the Creator.

A question can be asked to people who associate partners with Allah: What do they hope to gain from shirk? There is nothing in the heavens or the earths that can happen without the will of Allah. He is in control over everything, all the while not dependent on anything. Allah is the one who gives us sustenance, eyes to see, ears to hear, and a mind to ponder, yet some use these blessings to follow the cursed Satan into the depths of Hell by associating partners with Him.

One of the beautiful aspects of Islam is that there is no compulsion in religion. We are free to choose what and who we believe in, but as with any choice there are consequences. Deniers of Tawheed decide their own fate, and similar to the injustice they commit against Allah, they are unjust to themselves for rejecting the truth when it is presented to them. As for believers in the Oneness of Allah, the Mercy of Allah first touches their hearts with guidance from their Lord, and again when His promise of beautiful rewards are fulfilled.

And those who, when they commit an immorality or wrong themselves [by transgression], remember Allah and seek forgiveness for their sins - and who can forgive sins except Allah? - and [who] do not persist in what they have done while they know. Those - their reward is forgiveness from their Lord and gardens beneath which rivers flow [in Paradise], wherein they will abide eternally; and excellent is the reward of the [righteous] workers. [Quran 3:135-136]

Believing in Tawheed and rejecting shirk go hand in hand. We need to constantly purify our hearts to dispel the whispers of Satan, and beseech Allah for His help to perfect Tawheed by raising our hands and asking Him for guidance in our hearts, for our families, and our progeny’s sake, similar to how Ibrahim (Abraham) did:

And when Ibrahim said, "My Lord, make this city [Makkah] secure and keep me and my sons away from worshipping idols. [Quran 14:35]