The Qur'aan is for all of mankind
Say, "What thing is greatest in testimony?" Say, " Allah is witness between me and you. And this Qur'an was revealed to me that I may warn you thereby and whomever it reaches. Do you [truly] testify that with Allah there are other deities?" Say, "I will not testify [with you]." Say, "Indeed, He is but one God, and indeed, I am free of what you associate [with Him]."
Qur'aan 6:19
قوله تعالى: وأوحي إلي هذا القرآن لأنذركم به ومن بلغ
As for the saying of Allaah in the above Verse:
"...And this Qur'an was revealed to me that I may warn you thereby and whomever it reaches..."
صرح في هذه الآية الكريمة بأنه - صلى الله عليه وسلم - منذر لكل من بلغه هذا القرآن العظيم كائنا من كان،
Allaah has explicitly stated in this noble Verse that he sallallaahu alayhi wa sallam is a warner to each and every person to whom the Magnificent Qur'aan reaches, whoever he may be.
ويفهم من الآية أن الإنذار به عام لكل من بلغه،
It is understood from this Verse that the warning is general and for every one to whom the Qur'aan reaches
وأن كل من بلغه ولم يؤمن به فهو في النار،
And that whoever this Qur'aan reaches, and he does not believe in it, then he will be in the Fire
وهو كذلك
And this is how the matter is.
أما عموم إنذاره لكل من بلغه، فقد دلت عليه آيات أخر أيضا
As for the generality of the warning of the Qur'aan to everyone to whom it reaches, then other Verses also confirm this.
كقوله: قل يا أيها الناس إني رسول الله إليكم جميعا
Like His saying:
Say, [O Muhammad], "O mankind, indeed I am the Messenger of Allah to you all,
Qur'aan 7:158
وقوله: وما أرسلناك إلا كافة للناس
And His saying:
And We have not sent you except comprehensively to mankind as a bringer of good tidings and a warner. But most of the people do not know.
Qur'aan 34:28
وقوله: تبارك الذي نزل الفرقان على عبده ليكون للعالمين نذيرا
And His saying:
Blessed is He who sent down the Criterion upon His Servant that he may be to the worlds a warner -
Qur'aan 25:1
وأما دخول من لم يؤمن به النار،
As for the fact that those who do not believe in it will enter the Fire
فقد صرح به تعالى في قوله: ومن يكفر به من الأحزاب فالنار موعده
Then, Allaah has explicitly stated it in the following Verse
"...But whoever disbelieves in it from the [various] factions - the Fire is his promised destination..."
Qur'aan 11:Portion of Ayah 17
And we have the following hadiith:
"By the One in Whose hand is the soul of Muhammad, there is no one among this ummah, Jew or Christian, who hears of me than dies without believing in that with which I have been sent, but he will be one of the people of Hellfire."
Sahiih Muslim Hadith No. 218
وأما من لم تبلغه دعوة الرسول - صلى الله عليه وسلم - فله حكم أهل الفترة الذين لم يأتهم رسول، والله تعالى أعلم.
As for the ones to whom the da'wah of the Messenger of Allaah (sallallaahu alayhi wa sallam) does not reach, then the ruling regarding them is the same as the ruling regarding the people of al-fatrah (those who lived in the time when no Messenger came to them and were in a place where the call did not reach them).
These people will be tested on the Day of Judgment. And Allaah knows best.
Reference(s):
أضواء البيان في إيضاح القرآن بالقرآن
Volume 1 Page 476