The Definition of Tawhid
It is to single out Allah with Ar-Rububiyyah (Lordship), Al-Uluhiyyah (Godhood), and Al-Asma' was-Sifat (Names and Attributes). To believe that He is One in His Self and in His Attributes, that He is One in His Sovereignty and in His actions. Indeed, He is One in His Self, in His Names, and in His Attributes; there is none that is co-equal, comparable, or similar to Him.
There is nothing like Him, and He is the All-Hearer, the All-Seer (Surah Ash-Shura 42: 11)
He is One in His kingdom and actions; as Creator, Planner, and Sustainer of all that exists, He has no partner: Say: "0 Allah! Possessor of the kingdom, You give the kingdom to whom You will, and You take the kingdom from whom You will." (Surah Aal ‘Imran 3:26)
As the One Who deserves worship alone, He is One, for there is none that deserves worship but Him. Allah Almighty says:
Say: "Verily, I am commanded to worship Allah, making all religion purely for Him.” The Virtue of Tawhid
(Surah Az-Zumar 39: 11)
It is a great virtue to have belief in Tawhid; indeed, Allah made that belief to be a saving ship for His worshippers in this life and in the Hereafter.
As for this life - when one is from the people of Tawhid, living his life according to its implications, and not associating any partners with Allah whatsoever, Allah will shower him with safety, peace, guidance, and a good life, for He says: It is those who believe and confuse not their belief with wrong, for them (only) there is security and they are the guided. (Surah Al-An’am 6: 82)
"Wrong" here means to mix one's faith with Shirk. When one avoids that wrong, one will have achieved security and guidance. Allah Almighty says: Whoever works righteousness, whether male or female, while he (or she) is a true believer verily, to him We will give a good life. (Surah An-Nahl 16: 97)
As for the Hereafter - when one dies upon Tawhid and meets his Lord, not associating any partners with Him, He enters Paradise and Allah Almighty saves him from the Hellfire. In an authentic Hadith, in the two Sahihs it is recorded that the Prophet (Sallallahu Alayhi wa Sallam) said: Indeed, Allah has forbidden upon the Fire he who says, 'None has the right to be worshipped but Allah,' seeking from that Allah's Face.
The Shaikh mentioned that "there are three categories of Tawhid," a fact that the scholars derived through studying and contemplating verses of the Qur'an, the sayings of the Prophet (Sallallahu Alayhi wa Sallam) and the situation of the polytheists.
Of the three categories, the polytheists acknowledged two and rejected one, the one around which they disputed with the Messengers, fought with them, and showed enmity and hatred toward them, a fact that one appreciates when he reflects on the Qur'an, the life of the Prophet (Sallallahu Alayhi wa Sallam) and the history of all Messengers, peace be upon them.
Some have added a fourth category, called Tawhid Al-Mutaba'ah (Following) meaning that it is obligatory to follow the Messenger of Allah (Sallallahu Alayhi wa Sallam) and adhere to the Shari'ah. None is followed other than the
Prophet (Sallallahu Alayhi wa Sallam), for he is the greater Imam that is always followed. It is not permissible, therefore, to go outside of the boundaries of his Shari’ah, for the Shari'ah is one and its Imam is one. Every jinn and man must follow and submit to his Shari'ah and they must follow his way in Tawhid. Though this fourth category is understood, it really under the category called Tawhid Al-Uluhiyyah, for the Lord ordered His worshippers to follow the Book and the Sunnah, and owing the Sunnah is Tawhid Al-Mutaba'ah. Muslim scholars agree that it is obligatory to follow the way of the Messenger of Allah (Sallallahu Alayhi wa Sallam) and that no one is excused from following that way.
Then the Shaikh mentioned the three categories:
2) Tawhid Al-Uluhiyyah
3) Tawhid Al-Asma' was-Sifat