The answer can be approached in a single case, the case of Abu Talib, the uncle and foster-father of the Prophet (peace be upon him), who has not departed from the faith of his ancestors, while protected the Prophet in the most difficult times, and defended him against the greatest attacks. I quote a hadith about the death of Abu Talib:

Imam Bukhari relates a hadith in his Sahih:
Sa’eed ibn al-Musayyib narrates from his father: “When the time of the death of Abu Talib approached, Allah’s messenger (peace be upon him) went to him and found Abu Jahl ibn Hisham and ‘Abdullah ibn Abi Umayyah ibn al-Mughira by his side. Allah’s messenger (peace be upon him) said to Abu Talib: “O uncle!
Say: There is none worthy of worship besides Allah, a sentence with which I shall be a witness for you before Allah” Abu Jahl and ‘Abdullah ibn Abi Umayyah said, “O Abu Talib! Are you going to denounce the religion of Abdul Muttalib?” Allah’s messenger (peace be upon him) kept on inviting Abu Talib to say it (i.e. There is none worthy of worship besides Allah) while they (Abu Jahl and Abdullah) kept on repeating their statement until Abu Talib said as his last statement that he was on the religion of Abdul Muttalib and refused to say, There is none worthy of worship besides Allah.’ Then Allah’s messenger (peace be upon him) said, “I will keep on asking Allah’s forgiveness for you unless I am forbidden (by Allah) to do so.”

Related to this story reveled the two ayas of the Quran:

It is not fitting, for the Prophet and those who believe, that they should pray for forgiveness for Pagans, even though they be of kin, after it is clear to them that they are companions of the Fire. (Quran 9:113)

This is usually understood to refer to the prayer for the dead, - if they died unrepentant after Islam was preached to them, - if they actively resisted or opposed the Faith to the last. And - if the person praying knows that on account of deliberate contumacy the deceased may be said to have had the doors of mercy closed to him.

It is true thou wilt not be able to guide every one, whom thou lovest; but Allah guides those whom He will and He knows best those who receive guidance. (Quran 28:56)

The immediate occasion for this was the death of Abu Talib, an uncle whom the holy Prophet loved dearly and who had befriended and protected him. The Prophet was naturally anxious that he should die in the profession of the true Faith, but the pagan Quraish leaders persuaded him to remain true to the faith of his fathers. This was an occasion of disappointment and grief to the Prophet. Allah guides whom He wants or Allah guides the one who wants Him. Both interpretations are possible from the text.

One who has carefully read the hadith and the Quranic ayas sees that the decision is in Allah's hands.
Yet, over the past 1400 years, our scholars spent time and effort while formed two opinions. One says that Abu Talib goes to hell, no matter how much he supported the prophet. The other group believes that Abu Talib was muttering something in his last agony and that was probably the Shihadah, but no one knows for certain, so we cannot judge him. Both views are supported by numerous hadiths.
My opinion is to leave it to Allah! Don't you have enough to do in this world? If so, why do you go astray to an area where you can never get answer in your earthly life? Deal with your own business. If you do it well, at least you will probably go to Heaven and get your answer there.