You have an excellent point.
What about the people who died before him?
This is the Islamic version
Sheikh Nuh Keller dealt with this in the attached article [from:
http://www.masud.co.uk, a brilliant site], a refutation of the idea of the 'universal validity of all religions,' saying:
5. The Fate of Non-Muslims in the Afterlife
The reason that contemporary writers affected by the writings of Gunon and Schuon, such as Chittick and Gai Eaton (or such as Martin Lings, Titus Burckhardt etc.), seem to want the universal validity of all religions at any price, even to the extent of attributing it to masters like Muhyiddin ibn al-`Arabi ("in principle") or Emir `Abd al-Qadir ("he protected the Christians against massacre by taking them into his own home because he understood" [as if other scholars considered massacring them halal]) would seem to be the emotive impalatability of followers of other religions going to hell. Where is the mercy? Would Allah put someone in the hellfire merely for worshipping in another religion besides Islam? This question is answered by traditional Islam according to two possibilities:
(1) There are some peoples who have not been reached by the message of the Prophet of Islam (Allah bless him and give him peace) that we must worship the One God alone, associating nothing else with Him. Such people are innocent, and will not be punished no matter what they do. Allah says in surat al-Isra',
"We do not punish until We send a Messenger" (Koran 17:15).
These include, for example, Christians and others who lived in the period after the spread of the myth of Jesus godhood, until the time of the prophet Muhammad (Allah bless him and give him peace), who renewed the call to pure monotheism.
The great Muslim scholar, Imam Ghazali, includes in this category those who have only been reached with a distorted picture of the Messenger of Islam (Allah bless him and give him peace), presumably including many people in the West today who know nothing about Allah's religion but newspaper stories about Ayatollahs and mad Muslim bombers. Is it within such people's capacity to believe? In Ghazali's view, such people are excused until after they have had an opportunity to learn the undistorted truth about Islam (Ghazali: "Faysal al - tafriqa," Majmu'a rasa'il al-Imam al-Ghazali, 3.96). This of course does not alter our own obligation as Muslims to reach them with the da'wa.
(2) A second group of people consists of those who turn away from God's divine message of Islam, rejecting the command to make their worship God's alone; whether because of blindly imitating the religion of their ancestors, or for some other reason. These are people to whom God has sent a prophetic messenger and reached with His message, and to whom He has given hearing and an intellect with which to grasp it but after all this, persist in associating others with Allah, either by actually worshipping another, or by rejecting the laws brought by His messenger (Allah bless him and give him peace), which associates their own customs with His prerogative to be worshipped as He directs. Such people have violated God's rights, and have accepted to go to hell, which is precisely what His messengers have warned them of, so they have no excuse:
"Truly, Allah does not forgive that any be associated with Him; but He forgives what is less than that to whomever He wills" (Koran 4:48).
In either case, Allah's mercy exists, though for non-Muslims unreached by the message, it is a question of divine amnesty for their ignorance, not a confirmation of their religions validity. It is worth knowing the difference between these two things, for one's eternal fate depends on it.