Muslims should not judge with certainty that others will be dwellers of Hell
A:
Muslims do not say that non-Muslims will be in Hellfire unless they reject the message of Islam and Qur'an after it has been conveyed to them by the callers of Islam through their many languages. Allah (Exalted be He) says:
...this Qur’ân has been revealed to me that I may therewith warn you and whomsoever it may reach.
Allah also says:
And We never punish until We have sent a Messenger (to give warning).
Those who receive the Da`wah and insist on their disbelief will be among the dwellers of Hellfire. This is according to what is mentioned in the previous two Ayahs. The Prophet (peace be upon him) also said:
By Him in whose Hand my soul is, every Jew or Christian of this Ummah (nation) who hears about me and dies unbelieving in me will be of the inhabitants of Hellfire.
This Hadith is reported by Muslim in his Sahih.
There are many Ayahs and Hadiths that confirm this meaning. (Part No. 2; Page No. 146) As for those whom the Da`wah of Islam did not reach in a way that proves the truthfulness of Islam and leaves no excuse for disbelief, their affairs will be up to Allah (Exalted be He). The soundest viewpoint of scholars in this regard is that Allah will put them to trial on the Day of Judgment. Whoever obeys Him will be admitted to paradise, and whoever disobeys Him will be admitted to Hellfire.
Al-Hafizh ibn Kathir (may Allah be merciful to him) has pointed out this point in his interpretation of the Saying of Allah (Exalted be He):
And We never punish until We have sent a Messenger (to give warning).
This issue was also discussed by
Ibn Al-Qayyim (may Allah be merciful to him) in his book "Tariq al-Hijratayn" under the title "Tabaqat Al-Mukallafin" (Classes of Legally Competent Persons." For more information in this regard, we advise you to read these two books.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.