Tauhid is the main subject of the Holy Quran. The meaning of Tawhid in its simplest form is:
There is no god but Allah.

Quran has repeated this simple meaning of Tawhid in 60 different manners. Some of them are being mentioned here.

There is no god but Allah. [37:35]
There is no god but He. [2:163]
There is no god but You [21:87]
There is no god but Me. [16:02]
Who is god except Allah. [03:61]
There is no god but the one God. [05:73]
You have no god other than Him. [07:65]
Never was with Him any other god. [23:91]
Allah is only one God. [04:171]
Your God is one God. [18:110]
Your God is only Allah. [20:98]
He is only one God. [06:19]
Your God is one God. [02:163]
Most surely your God is one. [37:04]
He: Allah, is one. [112:01]

Why Quran is repeating the same words in many different ways ?
This is just to make it very clear that the message of Tawhid is very simple, understandable, easily graspable and readily perceivable.

It is also to wash out completely all kinds of suspicions and doubts about the existence of God and leaves no room for anyone to pollute his mind with the filth of polytheism and
agnosticism.

After these awakening phrases, Quran then offers an unquestionable evidence and a most powerful proof of Tawhid - The Divine witness:

“Allah bears witness that there is no god but He, and so do the angels and, men of immense knowledge. He is the Executor of justice, the only God, the Mighty, the Wise”.(3:18)

In the above verse of the Holy Quran, Allah, Who knows everything, and Who speaks nothing but truth, and nothing is hidden from His sight, is testifying Himself His Existence and His absolute Oneness:
The signatory of Divine witness are the most pure and steadfast souls - The loyal angles and the most knowledgeable men. This is the most authentic witness of Tawhid.

In the light of such awareness and watchfulness, a man of average intellect can see God through his mind’s eye.