In this writing, I will present the views of Imam Khomeini, Shahid Mutaharri and Sayyid Munir al-Khabbaz on this matter.
Imam Khomeini (d. 1989):
Deficient disbelievers (qāṣir) are those who did not subscribe to God’s religion because external factors outside their control prevented them from doing so. These factors range from living in a closed environment in which the message of Islam did not reach them to not having the right means to investigate the truth. Naturally, they are excused from punishment on the day of Judgement.
Blameworthy disbelievers (muqaṣṣir), as the name suggests, did not follow a religion while they had the convenience to do so; there were not obstructed to learn and investigate or if they did pursue research on these matters, they refused to follow the truth out of animosity and hardheadedness. Naturally, they are not excused on the day of Judgement.
In line with this, the late Imam asserts that “most of the disbelievers are deficient except a small portion of them who will be blameworthy. As for their laymen this position is clear, for the possibility of falsity of their faith has not crossed their minds, rather they are certain in the validity of their creeds and the falsehood of all other religions. Similar to how the Muslim layman knows with conviction the truth of his faith and the falsehood of the rest without giving the possibility of it being otherwise, due to education and upbringing in an Islamic environment; in the same manner, with no difference from this perspective, the layman non-Muslim believes in the authenticity of his faith. And the certain person (qāṭiʿ) is excused in following what has given him certainty, and cannot be considered as a sinner and disobedient, nor punishing him for following his certainty is plausible.
As for their educated class, the common case amongst them is that due to teachings infused in them from their early childhood and upbringing in environments of disbelief, they have become convicted and believers in their false faiths, to the extent that whatever they receive in opposition of their beliefs they will reject it with their stringent and made-up minds. Therefore, the Jewish and Christian scholar is like the Muslim: he does not consider the proofs of other faiths as valid and this invalidity becomes a necessary belief for him. All due to the necessity he sees in the truth of his faith and thereof not giving attention and consideration to all that opposes it.
Yes, indeed, amongst them are those are blameworthy. Those who posited the possibility of falsity in their faith but neglected research and examination of proofs out of animosity or hardheadedness, as it was the case in early Islamic history with Jewish and Christian scholars (who coexisted and challenged the Prophet).
In short, the disbelievers are like the ignorant denomination of the Muslims (in the sense that) they have their deficient ones, which makes up the majority, and they have their blameworthy ones. This conclusion is supported by the intellect and principles of justice (attributed to the Divine).” 
Shahīd Muṭahharī (d. 1979):
In his book, Divine Justice under the heading ‘Good deeds of the disbelievers’, Shahīd presents an in-depth discussion about the fate of people after they die. After reviewing and criticizing two extreme views: the salvation of all humanity and the salvation of a very small minority, he presents his own which I will summarise here:
- Both paradise and hellfire have levels.
- Not all dwellers of paradise are people who initially entered therein nor do all people of hell will reside eternally therein.
- Performing good deeds without the intention of God and the hereafter, means those deeds will not ascend to the higher realm simply because they were never aimed in the direction of those realms to begin with. As a consequence, there will be no reward for them in the hereafter.
- Regardless of faith, Muslim or not, any good deed performed for the sake of God will be rewarded in the hereafter.
- The proportion of good deeds performed by non-Muslims which is in accordance with the Divine program of Islam such as helping other humans and the like, will be rewarded. Acts of worship not in line with Islam’s instructions are void and will reap no fruit.
- Evil deeds can erase the desirable effects of good deeds. Thus, the good deeds of non-Muslims can be erased if they reject the truths of Islam out of bias and obstinacy after being presented the truth of Islam.
- Muslims and all other true monotheists will experience torment in purgatory and long punishments on Day of Judgement for their indecencies and sins. In some cases, such as murdering an innocent soul, they may reside therein forever.
- Good deeds of non-believers who have ended up in hell fire may help them in the lessening of their punishment or its upliftment.
- In the view of Islamic sages such as Avicenna and Mulla Sadra, the majority of people who haven’t confessed to the truth are excusable and not culpable.
Sayyid Munīr al-Khabbāz
Firstly, we cannot specify who deserves the punishment of Hellfire and eternal suffering, simply because we do not hold the keys of paradise and hell in our hands.
Secondly, verses of the Quran (e.g. 41:46, 20:40) and reason demonstrate to us that it is incompatible with God’s state of utmost perfection to commit reprehensible acts, the most hated of them being unjust to his servants. Thus, we can say:
There is no difference between the person born and raised in an environment of kufr and disbelief and the person born and raised in an environment of imān and faith; there is no difference between the person who is surrounded with complementary factors aiding his belief in God and the person who does not enjoy these helps; ultimately, God will judge each servant according to his will and effort.
If a person chooses the path of disbelief or disobedience, to the extent his free will is involved in this choice he will be punished. And if he chooses the path of obedience and faith, he will receive reward in accordance with the involvement of his voluntary effort in that regard.
To generically suppose that a person who was raised in a religious environment and chose to be a disbeliever is per se more worthy of punishment than the person who lived in non-religious environment and chose kufr is not correct. Similarly, a person who is raised in a non-religious environment and chooses faith is not more worthy of reward per se than someone who is raised in a religious environment and chooses to continue with this. The ultimate judge is the extent to which our will and effort is involved in the decision-making process, not the factors that are outside of our control whether they be positive factors or negative factors in the direction of guidance. This is the reason why the verses of the Quran establish a link between punishment/reward and the person’s will (99:7-8, 13:11).
Additionally, when we say a person who chooses the path of disobedience and disbelief is worthy of punishment, it does not necessarily imply that this punishment will be realized. There is a difference between eligibility (istiḥqāq) for reward/punishment and its realization/actualization (fiʿlīyah). For example, someone who dies in the state of sinning then we usually say he is eligible for punishment; but there is still room to ask the question of whether he will actually receive this punishment or not? This is ultimately in the hands of God. Some may be blessed with His mercy and pardon, or they may have performed certain good deeds that will act in intercession (shafāʿah) for them on the day of Judgement.
Finally, given the criteria for the eligibility of punishment and reward is one’s will, effort and capacity, then if it is possible for an individual to compare all the ideologies and religions and choose for himself one of these faiths substantiating his choice with logical and academic proofs, or he surveys all these religions comparatively and chooses the one that is closest to truth with respect to his level of knowledge and intellectual capacity, then he will be judged keeping in mind his willpower, capacity and struggle in finding the truth. And if there is another person who is not able to compare all religions or the sects of a religion, or at worst case scenario due to his poor education background and intellectual inferiority, he is not even able to compare analytically between two sects, then surely Allah will judge each individual separately with respect to how much of intelligence and willpower he has endowed them. Therefore, it is incorrect to require every person to compare between all ideologies, religions and sects and thereof choose the path representing the truth. People differ in their abilities and on the day of Judgement each person will be judged according to his abilities. The important point to reinforce is that eligibility of punishment and reward returns to one’s will, ability and struggle in the way of truth. As for all other external factors, they cannot be given merit in God’s judgement of His servants.