The  greatest sin is to contradict the purpose of oneÂ’s creation. Prophet Muhammad  (pbuh) said that the gravest sin was: “To give God a partner though He created  you alone.”  Worshipping others beside  God is the only unforgivable sin, and essentially involves giving the  attributes of the Creator to a part or parts of the creation.

“They worship  beside God that which neither hurts them nor profits them, and they say:  these are our intercessors with God. Say: Would you inform God of that he  knows not in the heavens or in the earth? Praised be He and high exalted above  all that you associate with Him!”
(Quran 10:19)

“Say: Shall I  seek another than God for Lord, when He is Lord of all things?”
(Quran 6:165)

"Say: Can  you see yourselves, if the punishment of God comes upon you or the Hour comes  upon you, calling upon other then God? Do you then call to any other than  God? (Reply) if you are truthful."
(Quran 6:40)

“And lo! Of his persuasion verily was Abraham when  he came to his Lord with a pure heart; when he said unto his father and his  people; What is it that you worship? Is it a falsehood beside God that you  desire? What then is your opinion of the Lord of the Worlds? And he glanced a  glance at the stars. Then said: Lo! I feel sick! And they turned their backs  and went away from him (for fear of disease). Then turned he to their gods and  said: Will you not eat? What aileth you that you speak not? Then he attacked  them, striking with his right hand. And his people came toward him hastening.  He said: Worship that which you yourselves carve when God has created you and  what you make?”
(Quran 37:83-96)

“(Joseph said): O my two companions of the prison!  Are diverse lords better, or God, The One, The Irresistible? Those whom you  worship beside Him are but names you have named, you and your fathers. God  has revealed no sanction for them. The decision rests with God only, Who  commands you that you worship none save Him. That is right religion, but most  men know not.”
(Quran 12:39-40)

Indeed God has explained that the attributing of partners to Him may  take many forms. To atheists and materialists life has no purpose beyond the  fulfillment of their desires. Consequently, their desires also become the god  that they obey and submit to, instead of the One true God.

“Have you seen him who  chooses for his god his own lust? Would you then be guardian over him?”
(Quran  25:43)

Also, due to the corruption of the previous messages, people  have taken prophets and messengers as their gods! This is seen for example with  the Roman pagan influence over Christianity, and how the true message was  changed over hundreds of years according to the desires and whims of its  leaders. This reformation continues to occur today, and with all other  religions that came only for their time and people.

“They say: God has taken a son – glorified be He!  He hath no need! His is all that is in the heavens and all that is in the  earth. You have no warrant for this. Tell you concerning God that which you  know not?”
(Quran 10:69)

“The Messiah, son  of Mary, was no other than a messenger. Messengers had passed away before him.  And his mother was a saintly woman. And they both used to eat earthly food. See  how We make the revelations clear for them, and see how they are turned away!  Say: Serve you in place of God that which has no power to hurt nor benefit?  God it is who is the Hearer, the Knower. Say: O people of the Scripture!  Stress not in your religion other than the truth, and follow not the vain  desires of people who erred of old and led many astray and erred from a plain  road.”
(Quran 5:75-77)