Short answer
Mainstream Sunni belief is that Paradise is for those who die believing in Allah alone and accept His final messenger once the message has truly reached them. Those it never reached in a clear way are not punished without a messenger, and their case is with Allah. Muslims do not declare any specific person to be in Hell, and Allah is perfectly just and merciful.
Hanafi (Maturidi) theologians agree that one who clearly received the message of Islam and rejected it, dying on disbelief, is not among the people of Paradise. Allah does not wrong anyone, and a person the message did not reach is judged with justice; many Maturidis held such a person is still asked about belief in the one Creator, which sound reason can reach.
The four schools agree on this.
Explanation
This question is asked with real love, often by converts thinking of parents or friends. The answer has two parts that both matter.
The principle. The Quran says that a religion other than Islam will not be accepted in the Hereafter, and the Prophet ﷺ said that any Jew or Christian who hears of him and dies without believing in his message will be among the people of the Fire. Islam here means submission to Allah alone through the final message. Verse 2:62, which promises reward to believing Jews, Christians and Sabians, is explained by the scholars as those who followed their prophet before the coming of Muhammad ﷺ. The four schools and Sunni theologians agree on this principle.
The mercy and justice. Allah says He does not punish until He has sent a messenger. Hearing the word Islam on the news, or meeting a distorted picture of it, is not the same as the message truly reaching someone. Scholars discuss those who never heard, or heard only a false version, and many say they will be tested on the Day of Judgement. Good deeds of a non-Muslim are rewarded in this world, and Allah wrongs no one.
- We never say that a particular person is in Hell. Only Allah knows how someone died and what reached their heart.
- The right response is to live and share Islam well, and to pray for guidance for those we love.
See tawheed, shirk and kufr and Jannah in Islam.
The evidence
Quran, Surah Aal-i-Imraan 3:85
وَمَن يَبْتَغِ غَيْرَ ٱلْإِسْلَٰمِ دِينًۭا فَلَن يُقْبَلَ مِنْهُ وَهُوَ فِى ٱلْءَاخِرَةِ مِنَ ٱلْخَٰسِرِينَ ﴿٨٥﴾
And whoever desires other than Islam as religion - never will it be accepted from him, and he, in the Hereafter, will be among the losers.
Salvation in the Hereafter is through Islam.
Quran, Surah Al-Israa 17:15
مَّنِ ٱهْتَدَىٰ فَإِنَّمَا يَهْتَدِى لِنَفْسِهِۦ ۖ وَمَن ضَلَّ فَإِنَّمَا يَضِلُّ عَلَيْهَا ۚ وَلَا تَزِرُ وَازِرَةٌۭ وِزْرَ أُخْرَىٰ ۗ وَمَا كُنَّا مُعَذِّبِينَ حَتَّىٰ نَبْعَثَ رَسُولًۭا ﴿١٥﴾
Whoever is guided is only guided for [the benefit of] his soul. And whoever errs only errs against it. And no bearer of burdens will bear the burden of another. And never would We punish until We sent a messenger.
No one is punished before the message reaches them.
Sahih Muslim 153
Sahih... لا يسمع بي أحد من هذه الأمة يهودي ولا نصراني ثم يموت ولم يؤمن ...
"No one of this nation, Jew or Christian, hears of me and then dies without believing in what I was sent with, except that he will be among the people of the Fire."
The condition is hearing of the Prophet ﷺ and his message and then rejecting it.
Sahih Muslim 2808
Sahih... إن الله لا يظلم مؤمنا حسنة ...
"Allah does not wrong a believer for a single good deed; as for the disbeliever, he is given the reward of his good deeds in this world."
Allah's justice: good deeds are never lost.
Where the ruling is recorded
- al-Aqida al-Tahawiyya, the sections on faith and on not declaring individuals to be in Paradise or Hell. Ahl al-Sunnah do not testify that a specific person of the qibla is in Paradise or Hell; scholars apply the same caution to individuals whose final state is unknown.
- al-Fiqh al-Akbar, the section on faith and disbelief. The Hanafi creed on belief and its absence.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.