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.
 

Now that we have gone through the axiom of monotheism and accepted it as a logical and rational principle, we should note that this belief is much more complicated than it seems. In order to reach a firm belief in the existence of God and His Oneness, one has to follow each of its stages.

These degrees include “unity of the essence of Allah”, “unity of the attributes of Allah”, “unity in what Allah does” and “unity in worship”. The first three of these belong to the realm of theoretical monotheism, while the last one is among practical monotheism. Here we will have a brief look at each of these steps.

1. Unity of the Essence of Allah

This stage of monotheism is defined as believing that God is One and Unique in His essence. The very first thing that one assumes regarding Allah is that of His Self-dependence; He is a being who is not dependent on any other beings in any way, while everything depends on Him and seeks His help.

Secondly, one has to believe in His Creatorship; that He is the creator and the ultimate source of all the existing things, all things are from Him, and He is not from anything. Accordingly, He is called the First Cause. The unity of essence, thus, means that this being is not multiplicable, and there is nothing like it. Since the self-existing truth is one, then this world has only one source and one end; it has neither originated from various sources nor will return to various ones, the whole universe has one center, one pole, and orbit.

2. Unity of the Essence and Attributes of Allah

It is defined as the recognition that the essence and the attributes of Allah are identical and that His various attributes are not separate from each other. While the Unity, in essence, refers to God’s Oneness, denying the existence of any peer or like for Him, the Unity in attributes means the negation of any kind of multiplicity or plurality within His essence.

All the attributes implying perfection and beauty belong to Him, and they are not at all separate from Him. The separation of the essence from the attributes and their separation from each other are the characteristics of a limited being, while God is infinite and such characteristics cannot be ascribed to Him. Accordingly, Allah has attributes which are unlimited – like His own self – and identical with His essence.

3. Unity in What Allah Does

The Unity of His work is to recognize that the world with all its systems is the work of Allah alone and has originated from His will only. Nothing in this world has occurred on its account and independent from God; every agent, cause or power owes its existence and effect to Him.

As He has no partner in His essence, He has no peer in what He does either. While human beings, as one of these agents and causes, have control over their own actions and can influence their destiny, they are not at all out of God’s will and supervision; otherwise, we should consider them as God’s partners, which will ultimately deny God’s unity in essence.   

4. Unity in Worship

Reaching this stage is when one only worships God and has prepared himself for praying Allah, the One. Based on Islamic doctrine, the act of worship has its own degrees, the clearest of which is the performance of the rites for Allah’s glorification and exaltation.

According to Holy Quran, worship is not limited to the actual performance of prayer but includes any form of spiritual orientation and idealization; the person whose purpose and motivation in life only satisfies his own whim has, in fact, led his spiritual direction toward it and thus worshiped it instead of God. So, the unity in worship is to see Allah as the only one who is fit to be worshiped and obeyed unconditionally and regard Him as the only purpose and direction of one’s conduct.

What Is the Difference between Theoretical and Practical Monotheism?

The first three degrees are all theoretical and a matter of creed - they should be recognized and acknowledged, and require rational thinking- while the fourth one is practical and a necessity for all of our actions.

Moreover, theoretical monotheism gives an insight to perfection while practical monotheism is the actual movement toward it; it is only seeing God’s Oneness and finding knowledge about it while practical monotheism is experiencing and feeling these beliefs tangibly in all walks of life.

In other words, theoretical monotheism provides the basis and foundation of practical monotheism; on the other hand, without bringing those beliefs into action, they would become deficient and imperfect.