If man has only a nominal share in his acts because his free will does not possess creative ability, why do we read in the holy Qur'an that The Creator of the heavens and the earth takes Man's rebellion so seriously?

Unbelief, rebellion and sinfulness mean destruction, and complete destruction can result from an act of disobedience of a most nominal weight. Just as a vessel may sink due to the helms-man’s negligence, whilst the efforts of the whole crew can achieve nothing to save it. A man’s free will can cause great depredations as a result of unbelief and sinfulness, no matter how insignificant a share it actually has in the performance of acts. Unbelief is so great a sin by its nature that it reduces the whole universe to something of no value, and contradicts the witnessing to the Oneness of God by the whole of creation, and denies the manifestations of the Divine Names. It is, therefore, entirely proper that God should take unbelief so seriously and threaten unbelievers so severely in the name of the whole of creation and the Divine Names, and it is absolutely just to condemn them to the eternal torment of Hell. Since rebellious men cause vast corruption through unbelief and sinfulness, the believers are in great need of God’s succor against them. A mischievous boy can set a whole house on fire although it is guarded by ten strong men, and those guards may need to apply to the parents of that boy to prevent him from committing such a crime. So too do believers need the help of God against rebellious and sinful men.

In short: It is permissible for the believer to speak of Divine Destiny and man’s free will, when his faith is perfect and when he is mature enough to perceive that all creation including himself is at God’s disposal. He assumes his responsibilities because he has free will and glorifies God by proclaiming Him to be free of any involvement with his own sins. He never forgets that he is a servant of God, and accordingly tries to follow all the Divine Commandments. On the other hand, he remains thankful to God by ascribing to Divine Destiny any good acts he has been able to achieve, and never boasts of them. Also he finds the strength to bear all his misfortunes by discerning in them the role of Destiny.

However, it is not proper for anyone who neglects the Divine Commandments to argue about Divine Destiny and man’s free will, since his defective thinking will lead him to ascribe the whole of creation to causes and ignore God completely. He appropriates all his skills and good deeds, regarding them to have originated directly with himself, and attributes his defects and failures to Destiny. This view means the reduction of Divine Destiny and free will to a senseless trick played upon man by his own commanding self (nafs ammara) to mislead him to the point where he disregards and denies his religious duties.