Taken from “Furu-ul-Iman: Branches of Iman” by Mawlana Ashraf Ali Thanawi rahimahullah
Two meanings of oneness of Allah or monotheism (Tawheed) have been proved from the legislator of law of Shari’ah. I.e. Rasulullah salalahu alayhi wa salam
“No one is worth worshipping except Allah”
“No one is the purpose (of worship) except Allah.”
First meaning is quite clear.
Allah Ta’ala says in the Qur’an:
“O my companions of the prison! (I ask you): are many lords differing among themselves better, or Allah, the One, Supreme and Irresistible?” (12:39)
“Whatever ye worship apart from Him is nothing but names which ye have named-ye and your fathers-for which Allah hath sent down NO authority: The command is for none but Allah: He hath commanded that ye worship none but Him: that is the right religion, but Most men understand not.” (12:40)
Allah Ta’ala says:
“And they have been commanded no more than this: to worship Allah, offering Him sincere devotion, being True (in faith); to establish regular prayer; and to practice regular charity: that is the religion Right and Straight.”(98:5)
Holy Qur’an is full of such verses that this is the Tawheed by its deficiency or loss one becomes Kafir or Mushrik (polytheist) and has to stay in hell forever. This is never forgiven.
Allah Ta’ala says:
“Allah forgiveth not (the sin of) joining other gods with Him: but He forgiveth whom He pleaseth other sins than this: one who joins other gods with Allah, hath strayed far, far away (from the right.)” (4:116)
The proof of the second meaning is like this that Rasulullah salalahu alayhi wa salam described hypocrisy (Riya-i.e. showing off) as Shirk-e-Asghar (Infidelity of smaller degree). It is obvious that in hypocrisy things other than Allah are not being worshipped but they are the purpose of the worship.
(For example a person is not offering salaat regularly but when he sees somebody else, he starts offering salaat just to impress him that he is regularly offering salaat. This is hypocrisy-In this situation though he worships Allah in Salaat but his intention is not to please Allah but somebody else.) When anything else other than Allah is made the purpose of worship, it is called Shirk, then Tawheed whcih is the opposite of Shirk, will mean that the worship should be done solely for the sake of Allah only to please Allah) and there should not be slightest intention to please anybody else by one’s worship. This is the meaning of
“No one is the purpose of worship except Allah”
Now we quote that hadith in which hypocrisy has been labelled as shirk.
It is narrated from Mahmood-bin-Lubaid radiallahu anhu that Rasulullah salalahu alayhi wa salam said:
“Most dangerous thing, of which I am afraid about you is shirk-e-asghar (smaller degree of infidelity or polytheism). People asked as to what was Shirk-e-asghar.
Rasulullah salalahu alayhi wa salam said:
“Hypocrisy (Riya-showing off in worship) is the Shirk-e-Asghar.” (It has been narrated by Ahmad)
There are so many other Ahadith in this regard, quoted in Tafseer Mazhari at the end of commentary of Sura-Kahf. We are not quoting them here.
Ikhlas (sincerity) is lost if the second meaning of Tawheed is not achieved, on the basis of which (i.e. not having Ikhlas) one will deserve punishment but will not stay in Hell eternally.
In nutshell, we can say that Tawheed means that we should not only worship Allah but the purpose of our worship should be only to please Allah. If we have the first thing and not the second, we are liable to be punished in Aakhirah.