Allah (SWT) has informed us that He can forgive any sin except associating partners with Him (shirk). Throughout history, all Messengers from Him were sent to mankind with the same essential message – worship Allah (SWT) and do not associate partners with Him. Islam came to revive the concept of strict monotheism in humanity which had been changed with time. This is the original core of all religions. Allah (SWT), in His mercy, has placed the inclination of recognizing Him as the only God is in the subconscious self of each human being. Indeed, every soul of the son of Adam (AS) has testified to the fact that Allah (SWT) is its Lord, before it was created.
It is in the essential nature of the human soul that it recognizes itself as vulnerable and thus needs to worship. If it does not worship Allah (SWT), it will worship something else. Satan has vowed that it will keep humans away from the supreme goal of worshiping Allah (SWT). It does this by diverting their focus away from Allah (SWT) and towards other false gods which can be idols, elements of the nature, living or dead people be they messengers, religious reformers, saints, clergy, forefathers, national leaders, famous personalities, Satan, angels, jinn, money, power, fame, ego, personal desires, etc.
What is the broad meaning of “worshiping” something? By worshiping a thing, it means that one is submissive to it, reveres it, considers it all powerful, obeys it unconditionally, loves it the most, fears it the most, tries to please it, never try to displease it, prays/pleads to it, takes one’s rulings from it, always thinks about it, thinks that only it can provide resources/solutions to him, thinks that it is its ultimate benefactor, etc. If someone has any of the above elements for something other than Allah (SWT), it means that he is doing shirk (associating with Allah (SWT)).
What is the sharr (evil) in shirk? Does Allah (SWT) want everyone to only worship him because of His ego, nauzobillah? Allah (SWT) is perfect and does not have such human deficiencies. Everything in the creation worships Allah (SWT). Grass grows, planets move in orbits, tides rise and fall, the clouds bring rain, predators hunt, birds chirp, bees make honey, etc. In everything they do that they were created to do, they are worshiping and praising the glory of Allah (SWT). They have no choice in this. Only man has been created different. He has a special status in Creation. He can either fulfill its destiny -- rising to a status higher than that of the angels by recognizing Allah (SWT) and voluntarily submitting to Him, using his free will and intellect to do so; or he can go lower than the animals by choosing not to. As vice-regents of Allah (SWT) on the Earth, he is supposed to carry out this ultimate test. Will he realize the purpose of his existence and rise to his full human potential, living an inspired life of fulfilling all its natural responsibilities? That inspiration comes from the guidance from Allah (SWT) directly – from contemplation on Creation in light of the Scriptures and adopting the lifestyle (Sunnah) of the last Messenger He sent to guide humanity – Muhammad (SWAS). He guides and inspires only those who depend directly on Him, i.e. worship Him alone, i.e. do not do shirk. To be receptive to this inspiration, one must be free of sins; one’s heart and mind should be clean to be colored with the right thoughts.
By living a life of submission to only Allah (SWT), the human soul will penetrate the mysteries of Reality through this inspiration. It will long for seeing the complete “picture” of this perfect Reality which he will see in Jannah when he sees the “Face” of Allah (SWT). That is the supreme joy for which human souls were created for. Their level in Jannah will be based on the clarity and depth of the spiritual vision that they cultivated in their sojourn in the earthly life. Those that have done shirk will not have not have this vision as they followed and worshipped other Creation and did not develop any spiritual insights of their own. Thus their deluded souls need to be cleansed by eternal burning in Hellfire as they failed to fulfill the true purpose of their creation – to worship only Allah (SWT).
Be inspired … be original … be the human you were always intended to be.