Your God is surely One. (37:4)

The term Tawheed demands a lengthier translation, namely: “the realization-and-affirmation-of-Oneness.
The Unity of God, the Divine Unity, Unity in its most profound sense.

Allah is The Real, The Absolute. 
Allah is One in His Essence,
His Attributes and His Acts, 
Before the existence of the universe 
there was nothing but The Creator.

The Qur`an declares that “Nothing is like Him” (42:11).

Allah is omniscient and all-powerful.
All of Allah’s attributes belong to Him exclusively.
He is the first and there is no last but His oneness.
He is the First without anything before Him.
He is the Last without anything after Him.
The end is with Him alone, and He is the End.
He is All-Existing: with him there is no end.
Allah is now as He was before. He is Eternal.

Tawheed can be viewed from the perspectives of:
Tawheed al-Rububiyah – Unity of Lordship
Tawheed al- Uluhiyah – Unity of Worship
Tawheed al- Asma wa Sifaat – Unity of His Names and His Attributes and
Tawheed al Itabaa – Unity in the following the Prophet Muhammad (PBUH)

Tawheed undoubtedly is the central theme and message of the Qur`an,
the Book of Allah and it is repeated in many other verses.
The Arabs in Mecca and later the Jews in Medina inquired
from Allah’s Rasul  , who is Allah?

Allah revealed in Surah Ikhlas (112) as follows:

In the name Of Allah, The most Gracious, The Most Merciful.
Say, Allah is One,
Allah The Everlasting, The Eternal,
He has not given birth and was not born,
And no one is comparable to Him

In another verse, S 24:35, an image of spiritual
discernment of Allah is mentioned,

Allah is the Light of the heavens and the earth………..
Light upon Light! And Allah guides to His Light whom He will.

These verses confirm the Unitary belief,
which is the mainspring of Islam and the core of The Qur`an.

Further, only in total acquiescence in Allah’s good pleasure,
renunciation of one’s own will, and surrender to His will and
guidance is Tawheed made perfect.

Allah’s absolute unity is the beginning and the end of the
spiritual life for Muslims.

The Unity of Allah is a magnificent and a beautiful concept,
and Muslims reaffirm their adherence to it many times a day.
To acknowledge the fact of the “affirmation-of-Oneness”
renew your faith by frequent invocation of 
“La ilaha illa llah” [there is non worthy of worship accept Allah]
will be the means to eradicate Shirk [polytheism].

It is a powerful and effective concept for focusing and
organising one’s worldview and epitomising a religious
and psychological orientation and authentically expressive
of an important facet of the spirit of Islam.
 
Shirk – Polytheism
The opposite of Tawhid is expressed by the term Shirk (polytheism),
which conveys the notion of “sharing” or “partnerships.”

The word Sharik (plural shurakaa) means “partner” or “associate.”
It encompasses far more than the more blatant forms of idolatry
and denial of Allah’s Unity.

The term Shirk means associating partners with Allah,
and the related term mushrik is applied to someone guilty
of such polytheistic association.

Forms of Shirk:

Major forms of Shirk:

Shirk ad Du’a – invoking or supplicating to a
                false deity besides Allah.

Shirk al-Niyah wa Iraada wal Qasd – having the intention
                and determination to deliberately worship
                a deity other then Allah.

Shirk at- Ta’a – obeying any created being against the
                command of Allah.

Shirk al-Muhabbah – loving a created being or an object
                     more than Allah.

Hidden Shirk as:

Shirk al- khafi means the hidden, or covert,
 association of partners with Allah, the One and Only God.
 This is an insidious form of Shirk.

Minor forms of Shirk as:
Shirk ar- rea – carrying out religious acts for worldly gain,
                e.g. showing off.
Shirk at- Tasmee – swearing by other then Allah.

Allah says in the Noble Qur`an that Shirk
is the greatest sin. (S 31: 14)

He also says that He will forgive all other
sins if He wishes, except Shirk (S 4: 49)

Muslims must understand the term Shirk (polytheism)
and be conceptually and practically clear that it is
diametrically opposed to, Tawheed,
the affirmation to the Oneness of Allah.