Allah Almighty is the all-knowing all-wise creator. He has considered a unique way of perfection for every creature and then guided it to reach its excellence (20:50). Human beings, as autonomous creatures, are in real need of divine guidance to know how to live a perfect and purposeful life. According to Islamic beliefs, the Quran is the divine revelation to Prophet Muhammad PBUH&HP that contains necessary guidelines for human beings: "As We sent to you an Apostle from among yourselves, who recites to you Our signs, and purifies you, and teaches you the Book and wisdom, and teaches you what you did not know." (2:151). The question is that, is the Quran Allah's words? Let's see why the answer is: Yes. 

1. The Holy Quran and the Prophetic Narrations are Not of the Same Style

If the Quran was Prophet Muhammad's PBUH&HP words, his PBUH&HP narrations should have been of the same eloquence and beauty of the Quran. But that's not the case. The Quran is unique in style, expression system, and arrangement. And that's why no one has been able to bring even a chapter (Surah) similar to that of the Quran. As the Quran says: "And if you are in doubt concerning what We have sent down to Our servant, then bring a Surah like it, and invoke your helpers besides Allah, should you be truthful." (2:23). Although the prophetic narrations are articulate and eloquent, some of them have been imitated and distorted, unlike the Quran [1].

2. Prophet Muhammad PBUH&HP Never Claimed These Words to Be His

If the Quran were Prophet Muhammad's PBUH & HP words, it would have been logical for him to claim that these eloquent words were his. Consequently, he could have promoted his position, fascinated his opposers, and gathered more followers. But, he never intended to be considered and praised as a superior power. On the contrary, in one instance, someone came to him and was uneasy because of his magnificence and grandeur, he said: "Calm down! I am not a king" [2]. 

3. The Objection of Prophet Muhammad's PBUH&HP Oppposers Were not Answered Immediately  

In some of the Quranic verses, the objections and disagreements of Prophet's PBUH&HP opposers are answered. If the Quran was Prophet Muhammad's PBUH&HP words, all these verses should have been quickly revealed right after an objection was raised, or when he was insulted or oppressed. But, this was not always the case [1]. 

For example, the Qiblah at the time of Prophet Muhammad PBUH&HP was originally the Noble Sanctuary (Al-Aqsa Compound), similar to Judaism. That was a reason for Jews to mock Muslims and say that Muslims follow them; hence, they are superior. Prophet PBUH&HP waited 16 months for the divine revelation to let him change the Qiblah while he was disturbed by Jew's words. It was after such a long delay that he received a response: "turn your face towards the Holy Mosque, and wherever you may be, turn your faces towards it!" (2:144). If the Quran was Prophet Muhammad's PBUH&HP words, he could have changed the Qiblah much sooner and suffer less. 

4. Some Verses of the Quran Warn Prophet Muhammad PBUH&HP

In some of the verses of the Quran, Prophet Muhammad PBUH&HP is warned. In Surah Tawbah, he is told that: "May Allah excuse you! Why did you grant them to leave [to stay behind] before those who told the truth were evident to you, and you had ascertained the liars?" (9:43), or in another Ayat: "A prophet may not take captives until he has thoroughly decimated [the enemy] in the land." (8:67). Is it logical at all that someone blames himself and makes it public?

5. Prophet Muhammad PBUH & HP Was not Able to Read and Write

Prophet Muhammad PBUH&HP was not literate, and this is stated in the Quran and historical records. "You did not use to recite any scripture before it, nor did you write it with your right hand, for then the impugners would have been skeptical." (29:48). So, how can someone illiterate bring such eloquent words?

6. There is no Contradiction in the Quran

This holy book was revealed for 23 years. And something astonishing about the Quran is that there is no contradiction among the verses of it. If it was a human's words, there surely existed contradictions and errors in the Quran because human beings are forgetful, and for example, they forget what they told two years ago. But, this is not true about the Quran.

 References:

1. Allah's words
2. M. H. Hakimi, et al. "Al-Hayat," vol. 2, Translation: A. Aram, Iran, 1986.

    

Every Muslim agrees on the fact that the Holy Qur'an came down as a revelation to the Prophet Muhammad (PBUH&HP)  in Arabic as we see in the following two verses:

The Holy Quran 43:3  
"Surely We have made it an Arabic Qu'ran that you may understand."

The Holy Quran  12:2
"Surely We have revealed it-an Arabic Qu'ran-that you may understand."

Do I need to read the Holy Quran in Arabic or can I just read it in English?

Today with uncountable nationalities embracing the fold of Islam, many read the Holy Qu'ran in their native tongue and thereby pose different questions and arguments about reciting it in their own tongue.  

These arguments include the following:

1. It is not wajib (Obligatory) to read the Holy Quran in Arabic because it is not specifically mentioned in the Holy Qu'ran that we must read it in Arabic.

2. Non-Arab Muslims do not understand the Holy Quran in it's revealed language and since Islam is a way of life, then it is more important to understand the injunctions, clear signs, stories of the Holy Prophets (AS) of Allah to gain guidance from these instead of spending time; learning a language, reciting in that language that they believe does not benefit them and that the time in which they would be occupied in reciting a language that is, they feel, not of use to them, then this time although not exactly wasted can be put to better use in acquiring actual knowledge.  

Some argue that they can not learn  Arabic because they are not good with languages and others still insist that while they read the Holy Quranic interpretations in their native language in the holy month of Ramadan, they have actually completed an entire Quran that month. 

And a final argument given is the following:

"I do not understand at all why the Quran should be read in Arabic? Yes, Allah Almighty has said that we sent the Qur'an in Arabic for you to think about, but if we read the Qur'an in Persian, can we not think about it?"
So let's begin to answer these arguments by going directly to the Holy Qu'ran where the word Arabic is mentioned exactly ten times and see what Allah is telling us.

In the Holy Quran 12:2,
"Surely We have revealed it-an Arabic Quran-that you may understand."

In the Holy Quran 13:37,
And thus have We revealed it, a true judgment in Arabic, and if you follow their low desires after what has come to you of knowledge, you shall not have against Allah any guardian or a protector. 

In the Holy Quran 16:103,
And certainly, We know that they say: Only a  mortal teaches him.  The tongue of him whom they reproach is barbarous, and this is a clear Arabic tongue.

In the Holy Quran 19:97,
So We have only made it easy in your tongue that you may give good news thereby to those who guard (against evil) and warn thereby a vehemently contentious people.

In the Holy Quran 20: 113,
And thus have We sent it down an Arabic Quran, and have distinctively set forth therein of threats that they may guard (against evil) or that it may produce a reminder for them.

The Holy Quran 26:193,194,195 and 200
193. The Faithful Spirit has descended with it.
194. Upon your heart that you may be of the warners.
195. In plain Arabic language.
200. Thus have we made it enter into the hearts of the guilty.

The Holy Quran 39:28,
An Arabic Quran without any crookedness, that they may guard (against evil).

The Holy Quran 41:3,
A Book of which the verses are made plain, an Arabic Quran for a people who know.

The Holy Quran 42:7,
And thus have We revealed to you an Arabic Quran, that you may warn the mother city and those around it, and that you may give warning of the day of gathering together wherein is no doubt; a party shall be in the garden and (another) party in the burning fire.

The Holy Quran 46:12,
And before it, the Book of Musa was a guide and a mercy: and this is a Book verifying (it) in the Arabic language that it may warn those who are unjust and as good news for the doers of good.

As we see from the ten verses above, Allah has a specific message that he wants to imprint on our minds and hearts by the adjectives which he brings with the word Arabic in the same verses.  However, let us not rely on our own assumptions or interpretations as I can already hear the arguments that may arise that, yes, it may be that it was revealed in Arabic because the people of the time were Arabs and how could they understand another language but today we come from many different nations and tongues.   So let us continue by looking at a clear hadith from Imam Jafar as-Sadiq (as). 

Imam Sadiq (as) said: 
"Learn Arabic, which is the word of Allah, and Allah has spoken to His servants in that language (the meaning of the Qur'an), and speak your jaws in Arabic and pronounce the end of the words clearly".  (Al-Khesal. Vol 2. P 258) 
In this above tradition, we see that the words of Imam As-Sadiq (as) are not advising or recommending us towards an action or a behavior, rather the words are decisive - a command.  What is our Imam telling? 

He says: "Learn Arabic", these are final and conclusive words from the sixth Imam (as).  He further tells us that Arabic is the word of Allah, he does not say the Quran is the word of  Allah but Arabic itself.  He tells us that Allah has spoken to His servants in Arabic "That language" and finally, he explains the physical aspect of the recitation - how we should recite in Arabic? "Speak your jaws in Arabic and pronounce  the end of the words clearly."

The above tradition coming from the Sixth Imam himself should also instill into our hearts that there is a science to the recitation of the  Arabic Quran and when something has a science, then there is a cause and an effect.  By mentioning the jaws,  we see that there is some physical benefit for our actual body that comes from reciting the Holy Qur'an in Arabic but what more does he tell us about this recitation?
The following tradition, answers this question.

Imam Jafar As-Sadiq (as) said "Try your best to read the Qur'an correctly in true Arabic pronunciation in the same way that Arabs read it." 
p 270 The life of Imam Al-Sadiq (as) Allama Baqir Sharif al Qarashi.

By the mentioned verses of the Holy Quran and the clear tradition (without even touching on the benefits of Arabic), there should be no doubt left about the actual command from our Imams and the guidance in the words of Allah himself that although we must understand the Holy Quran (reading in our native tongue), still there is nothing that can replace the function and role of the word of Allah in His own language.