Have we really understood Tawheed?

Shaykh Saaleh ahl-Shaykh mentions in his explanation of Kashf ash-Shubuhaat (Clarification of the Doubts), the following story:

Shaykh-ul-Islam Muhammad ibn Abdul-Wahhaab(rahimahullah) was with his students and after having completed the study of his book 'Kitab at-Tawheed', he wished to start again the study of this book for the 3rd or 4th time.

His students said to him : "O Shaykh, we wish to study another book, a book of fiqh or hadith".
The Shaykh replied: "Why (do you wish to study another book)?".
His students: "We have in fact understood Tawheed, we want to study another science".
The Shaykh: "Wait, I shall give some thought to this".
A few days later, the shaykh came to the lesson with a troubled face. His students asked him the reason for it.
The shaykh explained that he'd come across something which angered him. His students asked what it was.
The Shaykh: "It has been related to me that the inhabitants of a house have sacrificed a rooster (for a Jinn) in front of their door. I have sent someone to confirm this affair".
Some time later, his students asked him about the affair that he had mentioned.
The Shaykh replied: "In fact, the inhabitants of this house did not sacrifice for other than Allaah, but it was someone who fornicated with his mother".
Shocked, his students exclaimed:"We seek refuge with Allaah! He fornicated with his mother? We seek refuge with Allaah! He fornicated with his mother...".

After mentioning this story, Shaykh Saleeh explained that Shaykh-ul-Islam told this story to show them that the statement "We understand Tawheed" is ignorance and is one of the greatest tricks of the Shaytaan.
In fact, they have made greater a great sin [fornicating with one's mother] than shirk which exits from Islam.
Indeed, their hearts were not angered when the shirk that exits from Islam was mentioned.

And this also happens today, when some ignorant people see great sins and are angered by them, but when they hear about Shirk al-Akbar, or see people commiting shirk like sacrificing for other than Allaah, etc. this does not move their hearts.

This proves their ignorance. It proves that they have not understood Tawheed.

Hadith Qudsi-Superiority of Monotheism

Narrated Abu Huraira (radi Allahu anhu) Allah's Messenger (salla Allahu alaihi wa sallam) said: Allah (Most high) said: Among all partners, I am the most dispensable with association. So anyone who performs a deed in which he ascribes to Me others; I will abandon him and his act of polytheism.(This Hadith is sound and reported by Muslim.)

Shirk (polytheism) is a sin that if its doer dies without repentance, he will never be forgiven. Verily, whosoever sets up partners in worship with Allah, Allah has forbidden Paradise for him; the Fire will be his abode. One whose intention is not pure will deserve Hell. Purity of intention is essential in all deeds.