Shortly speaking...
Shortly speaking, there is always a luminous point inside the man’s heart which is the nearest mean of his communication with the world of super nature and the shortest path toward Allah. Wrong teachings, negligence, and pride, specially at the time of health and affluence, draw some curtains over it, but the mass of calamities and terrible events tear these curtains, put the ashes away from it and that luminous point inside the man’s heart appears.
It was for this very reason that the great leaders of Islam used to guide those who were seriously doubtful in the subject of theology by this way. We have heard the story of the wandering man who was in doubt for the subject of theology and Imam Sadiq (as) guided him aright through this very natural and innate way. The man said: “O son of the Messenger of Allah (S)!
(Please) guide me to Allah that who He is, because the tempters have made me bewildered.” Then the Imam told him: “O’ servant of Allah! Have you ever embarked a ship?” The man answered he had.
“(Thus) let them be ungrateful for what We have given them and let them enjoy, but they shall soon know.” Polytheism is a kind of ungratefulness and ingratitude unto the favours of Allah. We must not be happy with those successes which are accompanied with blasphemy and ungratefulness, because they usually have an evil end.
After mentioning so many reasoning upon Monotheism and theology, this verse confronts the opponents with a sharp and serious threat when it says: “(Thus) let them be ungrateful for what We have given them and let them enjoy, but they shall soon know.” They will enjoy of the fleeting pleasures, but they shall know very soon what the end of their disbelief and polytheism will be, and what an evil fate they will have.
It is true that the appearance of the verse here seems to be a command to disbelief and denial of the Divine verses, but it is evident that its purpose is a warning.