In The Name of Allaah, The Most Merciful, The Bestower of Mercy
The Muslims must be acquainted with the knowledge of Tawheed and Shirk. Shirk [i.e. to associate partners to Allaah in worship] is the gravest sin that can be committed in disobedience to Allaah. The Prophets brought the greatest [aspect] of knowledge, which is the knowledge of Tawheed, and they warned against the greatest wrong doing, which is to associate partners to Allaah. [Allaah said]: [ إِنَّ ٱلشِّرۡكَ لَظُلۡمٌ عَظِيمٌ۬ – Verily! Associating partners to (Allaah in worship) is a great wrong indeed] [Surah Luqmaan’ Aayah 13]
This is an affair which many of the callers to Islaam have turn away from at present and do not give it importance. The knowledge of Tawheed is the first thing they snub and flee from; [and likewise] Shirk is the first thing they decline [to address] when seeking to guide the Ummah. Neither do they warn the people nor caution them against this greatest danger known to mankind [i.e. Shirk]. Tawheed is the greatest [knowledge] brought by Prophets as glad tidings, but they [i.e. many of the callers] do not convey it; rather the greatest affair to them is modern politics.
The knowledge of Tawheed is the affair by way of which this Ummah is distinguished [in all virtuous traits] – sincerity in worshipping Allaah alone, purifying all the Islamic [arenas, sources of knowledge, practices, views, thoughts, places, aims, goals etc] from the filth and impurity of Shirk. Neither is there anything more impure [i.e. corrupts the heart, deeds, manners, thoughts, ideas, intentions, goals, aims etc] nor filthier than shirk, so why do we not purity the Ummah from this impurity and filth? Why do those [many] callers to Islaam deliberately feign ignorance of this impurity, in which the people wander blindly and belittle its severity, even though there is nothing equal to its [filthiness and impurity]?
It is obligated on the Muslims to free themselves from the filth of shirk and sincerely worship Allaah alone, so that they become the cleanest people and purest [i.e. in creed, deeds, thoughts, views, intentions, goals, aims etc]. And through this, they will be distinguished [in every virtuous trait].
If the mosques of the Muslims in the Islamic world are filled with graves- even though the Messenger severely cursed those who turn graves into places of worship- and those [many] callers to Islaam have not taken any steps to confront this filthy practice, then there can be no betrayal and deception -against the ummah- greater than this. [Allaah said]: [إِنَّ ٱلَّذِينَ يَكۡتُمُونَ مَآ أَنزَلۡنَا مِنَ ٱلۡبَيِّنَـٰتِ وَٱلۡهُدَىٰ مِنۢ بَعۡدِ مَا بَيَّنَّـٰهُ لِلنَّاسِ فِى ٱلۡكِتَـٰبِۙ أُوْلَـٰٓٮِٕكَ يَلۡعَنُہُمُ ٱللَّهُ وَيَلۡعَنُہُمُ ٱللَّـٰعِنُونَ – Verily, those who conceal the clear proofs, evidences and the guidance, which We have sent down, after We have made it clear for the people in the Book, they are the ones cursed by Allah and cursed by the cursers. [Surah Al-Baqarah’ Aayah 159]
The greatest clear proofs are Tawheed and the guidance that will remove the people from shirk. Therefore, O callers to Islaam! Fear Allaah. Safeguard the Muslims from this greatest danger and raise them to the highest station in this worldly life – the station of Tawheed; for [indeed] there is no station loftier than the station of Tawheed. Tawheed is the greatest station in the entire universe and shirk is the lowest.
Source: Marhaban Yaa Taalibal Ilm’ pages 104-106. Abridged & slightly paraphrased