Shaikhul-Islaam Ibn Taymiyyah would teach this book from cover to cover. And once he would finish the book, he would go back to the beginning and he would start it all over again. And when this happened on a number of occasions, his students asked that another book be started, that they had learned this affair of at-Tawheed. They wanted to continue on to another affair.
The shaikh, willing to test them and their understanding, set up a test for them. And he said to them that on such and such day there will be no dars (lesson) because he had to travel to see someone who had fornicated with his own mother. And his students were astonished and surprised. That was clear by the voices they made.
When [the shaikh] returned, the students asked as to what happened to this individual. Shaikhul-Islaam said it was not as he had explained - that he had commited fornication with his own mother - but rather a man was commiting shirk by slaughtering a small animal in the name of other than Allaah.
When he explained this, the astonishment from his students left them. They were no longer astonished. So the astonishment that was present when they heard a man was commiting fornication with his own mother was great. But when they heard of someone commiting shirk with a small animal, the astonishment was absent, as though this action [shirk] was less serious than the former action [fornication with his mother].
Shaikhul-Islaam, through this test, knew that his students did not understand this affair of at-Tawheed, and he began the book all over again.
This story was taken from here.
Lesson: Shirk is the greatest and the only unforgivable sin if you die upon it. Let us not treat it lightly. Too often we Muslims are guilty of getting upset if we see homosexuality, intoxication or immorality, yet we don't show the same level of anger at acts of Shirk. This shows that we have not yet realized how severe a sin it is.
Remember, in general sins,we are either oppressing ourselves or other people, but with Shirk we are giving Allah's rights to other than Allah, and what can be a greater sin than that!