The Noble Qur'an clarifies a great number of subjects. In fact, not all the Qur'anic revelations have been understood as yet. But as time goes by, theological, intellectual, scientific, and historical evidence contributes to a better understanding of these revelations by mankind. The following points are just a few examples:
13.1 The Oneness of God the Exalted and His Divine system (tawhid)
The following ayat of the Holy Qur'an are selfâ€‘explanatory in defining the concept of tawhid. 
"Say: He Allah, is One. Allah is the Absolute Selfâ€‘Sufficient. He begets not, nor is He begotten, and there is not a single one to equate with Him." CXII
"O people of the book!" There hath come to you Our rasul, revealing to you much that you used to hide in the Book, and passing over much (that is now unnecessary). There hath come to you from Allah a (new) light and a perspicuous (mubin) Book. Wherewith Allah guideth all who seek His good pleasure to ways of peace and security, and leadeth them out of darkness, by His Will, unto the light, guideth them to a Path that is Straight. In blasphemy indeed are those who say that Allah is Christ, the son of Mary..." V:15â€‘19
"Allah has said: `Take not (for worship) two gods, for He is only One Allah. Then fear Me and Me alone ...'." XVI:51
13.2 The unity of all Divine religions; Islam as a concept
The concept of Islam â€‘ submission to the Will of Allah â€‘ is nothing new. It existed well before the development of Islam as a religion. The following ayatclarify that this concept was preached by all the major prophets of Allah. This is a proof of the Unity of all Divine religions, even though they are called differently by mankind.
"Behold! his (Ibraham's) Lord said to him: `Be a Muslim (submit yourself to My will)'. He (Ibraham) said: `I am a Muslim (submit myself) to the will of the Lord and Cherisher of the Universe'. And this was the legacy that IbrMam left to his sons, and so did Yacqub; `Oh my sons! Allah hath chosen the Faith for you, then die not except in the Faith of Islam (submission to the will of the Almighty Allah)'." II:131 & 132
"When ‘Isa (Masih) found unbelief on their part he said: `Who will be my helpers to (the work of) Allah?'Said the disciples: `We are Allah's helpers, we should believe in Allah, and do thou bear witness that we are Muslims'." III:52
"O ye who believe! Bow down, prostrate yourselves, and adore your Lord. And do good that ye may prosper. And strive in His cause as you ought to strive (with sincerity and under discipline). He has chosen you, and has imposed no difficulties on you in religion; it is the cult of your father, Ibrahim. It is He (Allah the Exalted) Who has named you Muslims, both before and in this (revelation); That the Apostle should be a witness for you and you be witness for mankind! So establish regular Prayer, give regular Charity, and hold fast to Allah! He is your Protector â€‘ the Best to protect and the Best to help!" XXII:77 & 78
This ayah has many important points but the one we need to concentrate on is: "It is He Who has named you Muslims, both before and in this (revelation)". This very clear and strong statement makes Islam distinctly different from all religions of the world in which people coin a name for their religion, normally after the person whom they follow. But when it comes to Islam, Allah the Exalted clearly says that He Himself selected this very profound name, for all His revelations, either before or during the time Muhammad (SA).
For the definition of the word Islam please refer to the Glossary.
"And remember Ibrdhim and Isma’il raised the foundation of the House (with this prayer): `Our Lord! Accept (this service) from us. For Thou art the Hearing, the Knowing'. Our Lord! make us Muslims, bowing to Thy (Will), and of our progeny a people Muslim, bowing to Thy (Will); and show us our places for the celebration of (due) rites; and turn unto us (in Mercy); for Thou art the Oft Returning, the Merciful." II:127 & 128
"The Religion before Allah is Islam (submission to His Will): Nor did the `people of the book' dissent therefrom except through envy of each other, after knowledge had come to them..." III:19
The above ayat clearly confirm that Islam as a concept existed from the early days of the din of Allah the Exalted, but it took centuries before this concept finally culminated as a religion by His Will through Muhammad (SA), and the most appropriate name, Islam, was explicitly selected by Almighty Allah for His din.
Clearly, the din before Exalted Allah is Islam (being in the state of sub mission to Allah the Exalted) and there is punishment, as shown later on, for those who have seen the Precious Qur'an and yet turn away from His din.
"Ibrahim was not a Jew nor a Christian; But he was true in Faith as a Muslim. And he joined not gods with Allah." III:67
"Say: We believe in Allah, and in what has been revealed to us and what was revealed to Ibrahim, Isma’il, Ishaq, Yacqub, and the Tribes, and in that which was given to Musa and ‘Isa (Masih) and the prophets, from their Lord. We make no distinction between one and another among them and we are Muslims to Him (to Him we submit). If anyone desires a Religion other than Islam (submission to the Almighty Allah), never will it be accepted of him; and in the Hereafter he will be one of the losers." III:84 & 85
These passages clearly indicate that Islam as a concept goes back to the early days,  and that this concept of total submission to Allah the Omni potent has also been preached by other prominent messengers of Allah the Exalted preceding the last prophet.
It is significant to realise that in the entire Qur'an the singular form "din"(Religion) appears about 70 times but the plural form, "adyan", not one single time. This clearly shows that there is only one din before Almighty Allah.
13.3 Islam as a religion
Now let us see how this concept of Islam (submission to the will of Allah) finally culminates in the form of a religion called Islam.
"...This day have those who reject Faith given up all hope of your religion. Yet fear them not but fear Me. This day I have perfected your religion for you, completed My Favour upon you and have chosen for you Islam as your Religion .... " V:3
It is quite remarkable that in this very last preceptive revelation to Muhammad (SA), Allah the Omnipotent declares, emphatically, that the religion founded from the early times and gradually augmented by various prophets is now perfect, is named Islam and is chosen for humankind by Almighty Allah Himself. With this important message for mankind, the preceptive wahy ends, only 81 days before the last prophet leaves this world, (SA).
13.4 Islam as a single ummah
"And verily this ummah of yours is a single ummah and I am your Lord (and Cherisher). Therefore fear Me (and no other)." XXIII:52
"Verily, this ummah of yours is a single ummah, and 1 am your Lord (and Cherisher). Therefore, serve Me (and no other). But (later generations) will cut off their affair (of unity), one from another. (Yet) they will all return to Us." XXI:92 & 93
In his well known English translation of the Qur' an, Yusuf ‘Ali wrote:
"God's message was and ever is one; and his Messengers treated it as one. It is people of narrower views who come later and trade on the earlier names, that break up the Message and the Brotherhood [ummah] into jarring sects and tribes." p 843, n. 2750
The Islamic ummah is only one in Exalted Allah's view. All the boundaries and differentiations are our own makings. These ayat and the translator's footnote tell us about the unity of Almighty Allah's Word from the beginning to the end, the unity of His din, mankind, and the entire universe.
13.5 The universality of Islam
"We have not sent thee but as a Universal (Messenger) to men, giving them glad tidings, and warning them (against sin), but most men understand not." XXXIV:28
This is a very clear statement of the universality of Almighty Allah's Religion (the concept and religion of Islam). There are more to follow:
"Indeed We sent Nuh to his people, and he said: O my people! Worship Allah. Ye have no other god except Him. Verily! I fear for you the Punishment of a Dreadful Day" (VII:59) "And unto (the tribe of) ‘Ad (We sent) their brother Hud. He said O my people! Worship Allah. You have no other god save Him. Will you not ward off (evil)?" (VII:65) "And unto Madyan (We sent) their brother Shu’ayb. He said O my people! Worship Allah. You have no other god but Him." (VII:85) "And to (the tribe of) Thamud We sent their brother Salih. He said O my people! Worship Allah. You have no other god save Him." (VII:73) "And verily We have sent among every nation an apostle (proclaiming): Worship Allah and eschew evil." (XVI:36)
"And We sent Nuh to his people, so he remained among them a thousand years less fifty" XXIX:14
"And Ibrahim said to his people "Serve Allah and fear Him. That will be best for you, if ye understand! " XXIX:16
Every prophet was sent unto his own nation for their guidance, but the message of The Last Prophet Muhammad (SA) is general for all mankind:
"Say: O mankind! Verily! I am sent as the rasul to you all." VII:158
"Above all, it is a Message to all the worlds." LXVIII:52
Therefore, it is obvious that Almighty Allah's Religion had entered a universal level by the time Islam was introduced as a way of life (din) and that the Noble Qur'an is a Book of Guidance for all humankind, in all places and for all times.
13.6 Confirmation of previous revelations
"It is He Who sent down to thee (step by step), in truth, the Book, confirming what went before it. And He sent down the Tawrah (to Musa)and the Injal (to cIsa Masah) before this as a guide to mankind. And He sent down alâ€‘Furqan, the Criterion (of judgment between right and wrong â€‘ meaning the Holy Qur an)." III:3
"Nothing is said to thee that was not said to the messengers before thee..." XLI:43
13.7 Correction of people's misunderstanding and misbelief
"They say: Allah hath begotten a son'. Glory be to Him, Nay; to Him belongs all that is in the heavens and on earth. All shall render worship to Him." II:116