Bismillahir Rahmanir Raheem,
As-Salaamu 'Alaykum Wa RAhmatullahi Wa Barakatuh,
If you would like a comprehensive answer then you have to be precise in your wording.
Before you can get an answer we need to understand what the words in the question means. For example, Muslim and Islam. If a person says will only Muslims go to heaven, does he mean by Muslims, those who follow Muhammad and no one before? Or does he mean even the people before who may be called Muslims, like Jews and Christians who followed in the right way Moses and Jesus respectively?
Moreover, are the people who did not recieve a message Muslims?
I will leave this for someone more knowledgeable to give you the writing of scholars, but I will state a principle in Islam:
Allah does not punish until he has sent a messenger, i.e. until evidence is brought, and I am not giving a ruling (fatwa) rather passing you knowledge from Ibn Kathir,
rahimahullah,:
(Every time a group is cast therein, its keepers will ask: "Did no warner come to you'' They will say: "Yes, indeed a warner did come to us, but we rejected him and said: `Allah never sent down anything; you are only in great error.''') In these Ayat Allah reminds of His justice in dealing with His creatures and that He does not punish anyone until the proof has been established against them and a Messenger has been sent to them. This is similar to Allah's statement,
[وَمَا كُنَّا مُعَذِّبِينَ حَتَّى نَبْعَثَ رَسُولاً]
(And We never punish until We have sent a Messenger.) [17:15]
From the Tafsir of Ibn Kathir
http://tafsir.com/default.asp?sid=67&tid=54549 You may also read surah 67.
And Allah knows best
br.al-Habeshi