We were previously looking at what a few great Imamī scholars had to say on the issue of all Muslims reaching salvation, or more appropriately, the salvation of mankind completely. Yes, everyone, with the handful of exceptions, as the Qur’ān explicitly says: “And they denied the signs, though their souls acknowledged them”. So these people spoken about had certainty in what was true, and then they denied that. The word here used in Arabic refers to someone who rejects what they have established and accepted to be true.
This person has accepted in his heart that such and such an issue is correct, yet despite this certainty, he acts rebelliously against it. Of course, such a person shall be punished accordingly. On the contrary, who so ever does not act rebelliously against something they have accepted to be true, irrespective of the religion or custom they adhere to, shall reach salvation. The only exception to this would be if the individual acted in contrary to whatever their religion, custom, society or sect considered forbidden, in such an instance they shall be punished. On what basis? On the basis that they believed that those actions were forbidden yet they still performed them. Just like you and I.
You and I, as followers of the school of the Ahlulbayt, if we commit an act which is considered (within this school) to be forbidden we will be held accountable for that except where we are fortunate to receive intercession from God or the Prophet and his Family. This is in relation to us and our actions, what about all the other people from various religions? Just before we continue this discussion I don’t want to get to get too bogged down with terminologies, as to who is considered culpable (muqassir) and non-culpable (qāsir), as the jurists have differed with each other in relation to its definition. To give a general reminder, that if a person has a certainty providing proof and acts according to what the proof proves, irrespective of it being correct in reality, he will reach salvation.
This is a fundamental principle, that there exists a certainty producing proof and that a persons actions reflect what he believes. And that he is truthful to what he believes and acts sincerely in accordance to it. Such a person will be saved on the Day of Judgement. It isn’t important whether the person was amongst the people of Truth or not. Neither is it important whichever religion or faith he comes from, and whether the practices he adheres to come from a Divine message (like the Abrahamic faiths) or they don’t.
Syed Khomeinī in his jurisprudential discussion on Makāsib al-Muharrama brings up this discussion when mentioning the ruling of a transaction (with a non-Muslim) which involves harām money. What is the verdict on the non-Muslim laymen who follow other religions, like Judaism or Christianity? He says:
اما عوامهم فظاهر، لعدم انقداح خلاف ما هم عليه من المذاهب في اذهانهم بل هم قاطعون بصحة مذهبهم وبطلان ساير المذاهب
As for their laymen, it is apparent that [beliefs to] the contrary of what they believe in do not occur to them, on the contrary, they are absolutely certain that their religion is correct and all the others are false. 
Here obviously the laymen being referred to are those who aren’t scholars. Here Syed Khomeinī is saying that all of the Christian laymen, all of the Jewish laymen will reach salvation. For what reason? Because they believe and are convinced that they have the truth, and that this truth cannot be found anywhere else but their religion. And this is exactly the same with the Muslim laymen, (who believe that truth is absolutely with them and cannot be found elsewhere). He continues:
نظير عوام المسلمين، فكما ان عوامنا عالمون بصحة مذهبهم وبطلان ساير المذاهب من غير انقداح خلاف في اذهانهم لاجل التلقين والنشو في محيط الاسلام، كذلك عوامهم من غير فرق بينهما من هذه الجهة، والقاطع معذور في متابعة قطعه ولايكون عاصيا وآثما ولا تصح عقوبته في متابعته
Just like the Muslim laymen, our laymen know that their religion is correct and that all others are false. It doesn’t even occur to them [that they may be wrong]. This is due to the indoctrination [they have had] and [also due to] growing up within an Islamic environment. The laymen of other religions are the same, and there is no difference between them in this respect. The person who is certain [of being with the Truth] is exonerated in acting upon his certainty. He is not a sinner or a transgressor, and neither will he be punished in following his certainty. 
This is precisely the fundamental principle I have been mentioning, that it is irrelevant whether a person acts in according to reality or not, and on top of this he will not be punished for doing so. Now you might be thinking, that’s fine for their laymen but what about their scholars? What about the Buddhist scholars, the Sunni scholars, the Wahhabi scholars, scholars from whatever religion or creed you can think of, what’s the verdict on them?
واما غير عوامهم فالغالب فيهم انه بواسطة التلقينات من اول الطفولية والنشو في محيط الكفر: صاروا جازمين ومعتقدين بمذاهبهم الباطلة بحيث كل ما ورد على خلافها ردوها بعقولهم المجبولة على خلاف الحق من بدو نشوهم، فالعالم اليهودي و النصراني كالعالم المسلم لا يرى حجة الغير صحيحة وصار بطلانها كالضروري له، لكون صحة مذهبه ضرورية لديه لا يحتمل خلافه
As for their scholars, due to the indoctrination, they have had since childhood and from living in an environment of disbelief, the majority of them have become certain and believe in their religion [as the Truth] and see all other religions as false. Because of this, everything they see that contradicts their religion they refute it with their intellects that had been fashion from their early age in opposition to the Truth. The Jewish scholar and the Christian scholar is just like the Muslim scholar, they do not consider the proof of other religions to be correct, and they reject the proof [against them] like it was essential to them, on the basis that their own religion is absolutely correct and there is no option to entertain [ideas to] the contrary of it. 
This is what Syed Khomeinī has to say on this. He also mentions the “majority” of them (like I mentioned in the beginning). Why? Because (just like the laymen) they too consider that the Truth is in their religion, their jurisprudence, their way of thinking and their creed. I would add to what Syed Khomeinī has said and said that even the Buddhist scholars, the Sunni scholars, the Ash’ari scholars etc, all of them are to be exonerated (ma’dhῡr).
Unfortunately, you’ll find in the works of some scholars this strange notion that when a person reaches certainty (yaqīn) they have automatically acted in accordance with what is True in reality. It would appear that these scholars haven’t read the basics of epistemology, as if that was the case (that everyone who acted on certainty also acted in accordance with what is True in reality), you would get a contradiction. Is it possible that a Jewish person reaches certainty on one thing and I reach certainty on the complete opposite yet both of us are acting in according to reality? This is absurd! Only one person can be right, but since both have certainty, they are both exonerated. This is known as the Principle of Certainty. Unless a person has a considerable level of doubt in a matter that the Truth may be in another religion yet ignores it arrogantly and dogmatically, then such a person would be held accountable on the Day of Judgement.
So to conclude, these were the words of another great Imāmi scholar which were completely coherent with the idea that I have been mentioning of the permissibility in acting according to any Islamic madhab, in fact the permissibility in acting according to any Divine religion, no, in fact any path or creed whatsoever, so long as the three conditions mentioned previously are adhered to.
1 – Qur’ān, Surah 27, Verse 14
2 – Al-Makāsib al-Muharrama by Syed Khomeinī, v. 1, p. 133
3 – ibid
4 – ibid