Evidence of the Oneness of Allah
Question

Is it possible to give the Mushrikun (polytheists, those who associate others with Allah) proof of the oneness of Allah?
Praise be to Allah.
The entire universe, in the way it is created and the way it is controlled, bears witness to the oneness of Allah :

{Surely, His is the creation and commandment. Blessed is Allah, the Lord of the `Alamin (mankind, jinn and all that exists)} [Al-A`raf 7:54 – interpretation of the meaning]

The creation of the heavens and the earth , the alternation of night and day, the different kinds of inanimate objects, plants and fruits, the creation of humans and animals… all of that indicates that the Almighty Creator is One with no partner or associate:

{That is Allah, your Lord, the Creator of all things, La ilaha illa Huwa (none has the right to be worshipped but He). How then are you turning away (from Allah, by worshipping others instead of Him)?} [Ghafir 40:62 – interpretation of the meaning]

The variety and greatness of these created things, the perfect way in which they are formed, the way in which they are maintained and controlled, all indicate that the Creator is One and that He does as He wills and rules and decides as He wants:

{Allah is the Creator of all things, and He is the Wakil (Trustee, Disposer of affairs, Guardian) over all things.} [Az-Zumar 39:62 – interpretation of the meaning]
All of the above indicate that this universe has a Creator, that this dominion has a Sovereign, that behind these forms is One Who gives them shape:
{He is Allah, the Creator, the Inventor of all things, the Bestower of forms. To Him belong the Best Names.} [Al-Hashr 59:24 – interpretation of the meaning]

The soundness of the heavens and the earth , the order of the universe, the harmony among created things, all indicate that the Creator is One, with no partner or associate:

{Had there been therein (in the heavens and the earth) gods besides Allah, then verily, both would have been ruined. Glorified be Allah, the Lord of the Throne, (High is He) above all that (evil) they associate with Him!} [Al-Anbiya’ 21:22 – interpretation of the meaning]

These great created things either created themselves – which is impossible – or man created himself then created them – which is also impossible:

{Were they created by nothing? Or were they themselves the creators? Or did they create the heavens and the earth? Nay, but they have no firm Belief.} [At-Tur 52:35-36 – interpretation of the meaning]

Reason, Revelation and the original state of man (Fitrah ) all indicate that this universe has One Who brought it into being, that these created things have a Creator Who is Ever-Living and Eternal, All-Knowing and All-Aware, Powerful and Almighty, Kind and Most Merciful. He has the Most Beautiful Names and Sublime Attributes and He knows all things. Nothing is beyond His control, and there is nothing like unto Him:

{And your God is One God (Allah), La ilaha illa Huwa (there is none who has the right to be worshipped but He), the Most Gracious, the Most Merciful.} [Al-Baqarah 2:163 – interpretation of the meaning]
The existence of Allah is something that is there is no excuse for not knowing. It is the matter of common sense:
{Their Messengers said: What! Can there be a doubt about Allah, the Creator of the heavens and the earth?} [Ibrahim 14:10 – interpretation of the meaning]
Allah has created people with the natural instinct (Fitrah) to affirm His Lordship and Oneness, but the devils came to the sons of Adam and caused them to deviate from their religion.
According to a Hadith Qudsi (a type of prophetic narration), “I created all My slaves as Hanifs (monotheists), but the devils came to them and made them deviate from their religion, and they forbade them that which I had permitted to them.” (Narrated by Muslim, 2865)
Among them are those who deny the existence of Allah, and those who worship the devil, and those who worship man. And there are those who worship the dinar [money], or fire, or genital organs, or animals. And there are some who associate in worship with Him a stone from the earth or a star in the sky.
These things which are worshipped instead of Allah, cannot create or provide; they cannot hear or see, or bring benefits or cause harm. So how can they worship them instead of Allah?
{Are many different lords (gods) better or Allah, the One, the Irresistible?} [Yusuf 12:39 – interpretation of the meaning]
Allah has condemned those who worship these idols which cannot hear, see or think:
{Verily, those whom you call upon besides Allah are slaves like you. So call upon them and let them answer you if you are truthful. Have they feet wherewith they walk? Or have they hands wherewith they hold? Or have they eyes wherewith they see? Or have they ears wherewith they hear?} [Al-A`raf 7:194-195 – interpretation of the meaning]
{Say (O Muhammad to mankind): ‘How do you worship besides Allah something which has no power either to harm or benefit you? But it is Allah Who is the All-Hearer, All-Knower} [Al-Ma’idah 5:76 – interpretation of the meaning]
How ignorant man is of his Lord Who created him and provides for him. How he rejects Him and forgets Him, and worships others instead of Him:
{Verily, it is not the eyes that grow blind, but it is the hearts which are in the breasts that grow blind.} [Al-Hajj 22:46 – interpretation of the meaning]
Glorified be Allah far above that which they associate with Him. Praise be to Allah the Lord of the Worlds:
{Say (O Muhammad): Praise and thanks be to Allah, and peace be on His slaves whom He has chosen (for His Message)! Is Allah better, or (all) that you ascribe as partners (to Him)?” (Of course, Allah is Better).
Is not He (better than your gods) Who created the heavens and the earth, and sends down for you water (rain) from the sky, whereby We cause to grow wonderful gardens full of beauty and delight? It is not in your ability to cause the growth of their trees. Is there any god with Allah? Nay, but they are a people who ascribe equals (to Him)!
Is not He (better than your gods) Who has made the earth as a fixed abode, and has placed rivers in its midst, and has placed firm mountains therein, and has set a barrier between the two seas (of salt and sweet water)? Is there any god with Allah? Nay, but most of them know not!
Is not He (better than your gods) Who responds to the distressed one, when he calls on Him, and Who removes the evil, and makes you inheritors of the earth, generations after generations? Is there any god with Allah? Little is that you remember!
Is not He (better than your gods) Who guides you in the darkness of the land and the sea, and Who sends the winds as heralds of glad tidings, going before His Mercy (rain)? Is there any god with Allah? High Exalted be Allah above all that they associate as partners (to Him)!
Is not He (better than your so-called gods) Who originates creation, and shall thereafter repeat it, and Who provides for you from heaven and earth? Is there any god with Allah? Say: ‘Bring forth your proofs, if you are truthful’} [Al-Naml 27:59-64 – interpretation of the meaning]
And Allah knows best.