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.
 

“Muhammad is not the father of any man among you, but he is the Apostle of Allah and the Seal of the Prophets, and Allah has knowledge of all things” (33:40).

Based on some Islamic narrations, God has sent as many as 124000 prophets to humankind throughout history in different lands; the first of whom was Prophet Adam (AS), and the last divine prophet was Prophet Muhammad (PBUH).

Why Did God Send Prophets?

What do you think was the primary purpose for which the prophets were raised? God says in the Holy Quran:

“O Prophet! Indeed, We have sent you as a witness, as a bearer of good news and as a warner. and as a summoner to Allah by His permission, and as a radiant lamp” (33:45-46)

“Certainly We sent Our apostles with manifest proofs, and We sent down with them the Book and the Balance, so that mankind may maintain justice;” (57:25)

According to the verses above and many other verses in the Quran [i], prophets were all sent to awaken the inner prophet within each person (i.e. people’s reason), guide them to the right path and invite them to monotheism (worshipping the only One God), which would ultimately lead to their well-being and salvation here in this world and also the hereafter. In fact, no one can deny the effective role prophets had in different aspects of people’s lives in history, as Will Durant puts:

“There is no significant example in history, before our time, of a society successfully maintaining moral life without the aid of religion. France, the United States, and some other nations have divorced their governments from all churches, but they have had the help of religion in keeping social order” [1].

Did Different Prophets Bring Different Religions?

Now you might wonder, “if God is One and all the prophets were sent to bring us closer to Him, then why would He send different religions?”

From the Quranic point of view there is No such thing as different religions, but rather “all Prophets irrespective of the fact whether they had or did not have an independent code of law, had the same mission and preached the same message” [2]. Quran says:

“Indeed, with Allah religion is Islam, and those who were given the Book did not differ except after knowledge had come to them…” (3:19)

So, why would God send as many as 124000 prophets?

The answer is God’s message had to be renewed and developed according to the requirements of the time, the environment and also the people to whom God’s prophets were sent. Based on the stage of human development in each time and environment, a preaching or law-giving prophet [ii] would come to restore the divine message which had previously been changed, destroyed or distorted by people.

Was Prophet Muhammad (PBUH&HP) The Last Prophet?

As stated above, people’s intellectual immaturity was the main reason why God renewed his message and developed it over time. However, by the advent of Islam and revelation of the holy Quran, humanity had passed the period of its childhood and had become mature enough to preserve its intellectual heritage, i.e., the religion of God. One thing that proves human’s maturity at the time of Prophet Muhammad (PBUH&HP) is that literacy had become more common among people.

That is why the Prophet’s (PBUH&HP) main miracle is not of anything physical, material and time-dependent, instead, it is a book which is made of words understandable for people of all times. In other words, God, then, guides people and shows them His signs using words (Quran) not by events like parting the sea (led by prophet Moses) or healing the blind or the sick (led by prophet Jesus). God’s message, accordingly, was revived through the revelation of the holy Quran and hence, Muhammad (PBUH&HP) was the last prophet of God.

There Is No Need for a New Prophet

It is noteworthy that the majority of divine prophets were the preaching ones who would interpret and spread the teachings of the law-giving prophets. But now that God has provided all requirements for humans to achieve their individual and social perfection giving them His book of guidance and wisdom to think, and people have reached a higher intellectual stage and become mature enough to interpret God’s message [iii], there won’t be a need for a new prophet.

At this age of knowledge, the religious scholars are capable of applying the general principles of Islam to the requirements of the time and place and deducing the rules of religious laws [2]:

“The scholars are the inheritor of prophets” Imam Sadiq [4].

It is clear from what has been discussed above that the intellectual maturity of human has played an essential role in the finality of prophethood in several ways:

It has enabled man to receive his evolutionary program all at once and not by stages,
It has enabled him to keep the celestial Word of God (Quran) unaltered,
It has enabled him to interpret the revealed word of Allah, preach and propagate the teachings of religion and apply the religious principles to all the changing circumstances [2].

The last mentioned task is now performed by religious experts (Mujtahid) and is named Ijtihad meaning striving and making an effort to deduce the divine laws of Islam from the reliable sources and proofs.

Finally, …

The finality of prophethood does not mean that humanity is now able to dispense with religion. It is the need for prophets and the revelation of God that has ceased to exist, which is only because God’s message has been immune to any sort of alteration or distortion since the advent of Islam:

“Indeed We have sent down the Reminder, and indeed We will preserve it” (15:9)

This message is indestructible, and all people of next generations can have a recurring experience when dealing with it, irrespective of time.

However, God has never left the earth devoid of His guidance, and Imams as the successors of the Holy Prophet (PBUH&HP) have played the main part in maintaining humans’ spiritual connection with God from then on.

Notes:

[i] “Certainly We raised an apostle in every nation [to preach:] ‘Worship Allah, and shun fake deities.’ Among them were some whom Allah guided, and among them were some who deserved to be in error. So travel over the land and observe how was the fate of the deniers” (16:36). Also see: (7:59, 7:65, 7:85 and 7:73)

[ii] The messengers of God where either law-giving, who would be sent with a noble message and guidance and whose number did not exceed the fingers of one hand, or preaching who would spread and propagate the teachings of the law-giving ones.

[iii] "We ought to present you with the principles and you ought to arrive at specific conclusions from them." Imam Sadiq (AS) [3]

[iv] the holy Quran, the tradition of the holy prophet and 12 infallible Imams (Sunnah), consensus (Ijma`), and reason (`Aql)

Reference:

1. Mand and Universe
2. Wasail al-Shia, Bab 6, Hadith 51, 52
3. al-Kafi, vol. 1, p. 32, Hadith 2