Tawheed is the basis of the religion of Islam and is precisely expressed in Arabic language in the formula “LA ILAAHA IL-LALLAAH” (There is no God but Allah). This seemingly simple formula forms the dividing line between Iman (true belief in Allah) and Kufr (disbelief or denial). It literally means “unification” however when the term is used in reference to Allah, it implies DIVINE UNITY and combines the following three aspects.
         
It is the belief that

        (I)     Allah is One, and has no partners in His dominion and His actions.
       (II)    He is the One without similitude in His essence and attributes.
       (III)   He is One without rival in His Divine Unity and in worship.

The omission of any one of these attributes is referred to as “Shirk” (sharing) i.e. seeking association of a partner with Allah, which is idolatry and takes one out of realm of Islam, and a person with such a belief will find his place in hell. 

When the non-believers of Makkah, the home town of the Prophet, asked him what his God looked like and what material He was made of, whether He was made of gold, silver or something else, the Prophet was directed by Allah to quote to them in reply the following verses, which are part of the Qur’an.

“He is Allah, the One and Only. Allah is Independent of all and all are dependent on Him. Neither has He an offspring nor is He the offspring of anyone, and none is equal with Him in rank”. 

This verse, in a few words describes Allah most perfectly, in the sense that there is no one in the entire universe, nor ever was, nor ever can be, who is similar to Allah, or equal in rank with Him, or resembling Him in His attributes, actions and powers in any degree whatsoever. This sets the criterion of belief in Islam.  Worshipping someone else, may they be prophets or angels or forces of nature and making them idols is un-Islamic, and a sin not pardoned by Allah.