In The Name of Allaah, The Most Merciful, The Bestower of Mercy

Indeed Tawheed is the foundation upon which Islaam is founded.  It is its pre-requisite by way of which (one’s Islaam) is acknowledged (in the sight of Allaah) and by way of which good deeds are accepted.  By way of it sins are pardoned and the slave is entered into paradise and saved from the hell fire.  Due to (Tawheed), there occurred the dispute between the Messengers and the polytheists, and the swords of Jihaad were unsheathed.  Due to (Tawheed), the Jinn, mankind, paradise and hell fire were created; and righteous deeds are cancelled through that which nullifies (Tawheed) and that is Shirk.  Allaah (The Most High) said:

وَلَقَدْ أُوحِيَ إِلَيْكَ وَإِلَى الَّذِينَ مِن قَبْلِكَ لَئِنْ أَشْرَكْتَ لَيَحْبَطَنَّ عَمَلُكَ وَلَتَكُونَنَّ مِنَ الْخَاسِرِينَ

بَلِ اللَّهَ فَاعْبُدْ وَكُن مِّنَ الشَّاكِرِينَ

”And indeed it has been revealed to you (O Muhammad ), as it was to those (Allah’s Messengers) before you: “If you join others in worship with Allah, (then) surely (all) your deeds will be in vain, and you will certainly be among the losers.”  Nay! But worship Allah (Alone and none else), and be among the grateful.’’ [Soorah Az-Zumar: Aayaat 65-66]

All sins can be pardoned except shirk.  Allaah (The Most High) said: إِنَّ اللَّهَ لَا يَغْفِرُ أَن يُشْرَكَ بِهِ وَيَغْفِرُ مَا دُونَ ذَٰلِكَ لِمَن يَشَاءُ ۚ وَمَن يُشْرِكْ بِاللَّهِ فَقَدْ ضَلَّ ضَلَالًا بَعِيدًا

”Verily! Allah forgives not (the sin of) setting up partners in worship with Him, but He forgives whom he pleases sins other than that, and whoever sets up partners in worship with Allah, has indeed strayed far away.’’ [Soorah An-Nisaa: Ayah: 116]

And due to shirk the slave is prevented from entry into paradise and hell fire is imposed upon him for eternity.  Therefore for this reason giving concern to Tawheed is the most important affair amongst all important affairs and the greatest of all obligations.  Abandoning (Tawheed), turning away from it and from learning it is the greatest calamity.  And due to this it is incumbent upon every slave (of Allaah) to learn (Tawheed) and to learn that which nullifies and negates it, or that which diminishes it and infringes its rights.

For further details, see: Ash-Sharh Al-Moojaz, page: 5 onwards