I’ve been having a series of enlightening and interesting conversations with my brother in Najaf, and in the context of one, he brought up this report from one of the historic Shi’i hadith compilations, Usul al-Kafi, developed in approximately the ninth century, or three hundred some years after the Prophet’s death. I found it fascinating (and take sole responsibility for all conclusions I draw from it). الكافي ج 2, كتاب الإيمان والكفر, باب دعائم الإسلام, ح 5: علي بن إبراهيم ، عن أبيه وعبد الله بن الصلت جميعا ، عن حماد بن عيسى ، عنحريز بن عبد الله، عن زرارة ، عن أبي جعفر ( عليه السلام ) … قال : ثم قال (أبو جعفر): ذروة الامر وسنامه ومفتاحه وباب الأشياء ورضا الرحمن الطاعة للإمام بعد معرفته ، إن الله عز وجل يقول : ((من يطع الرسول فقد أطاع الله ومن تولى فما أرسلناك عليهم حفيظا)) أما لو أن رجلا قام ليله وصام نهاره وتصدق بجميع ماله وحج جميع دهره ولم يعرف ولاية ولي الله فيواليه ويكون جميع أعماله بدلالته إليه ، ما كان له على الله عز وجل حق في ثوابه ولا كان من أهل الايمان ، ثم قال : أولئك المحسن منهم يدخله الله الجنة بفضل رحمته.  النساء : 80
For the non-Arabic speakers, it reads in the part most relevant for our purposes as:
Then [Abu Ja’far] said, the essence of the matter, its climax, its key, the door to things and the pleasure of God is obedience to the Imam after he is known. And God the Mighty and Momentous has said “Who obeys the Apostle obeys God, and who turns away, We have not sent you as a protector.” (Nisa:81) But as for a man who stands his nights [in prayer] and fasts his days and has given all of his wealth in charity, and has done the Hajj in all periods, and he did not know the sovereignty of the one near to God [i.e. the Imam], to follow him and have all of his deeds be through his guidance, there is no right to him for repentance from God the Mighty and Momentous, and he is not of the people of faith.
Then he said, He is a good person among them, and Allah shall enter him into Paradise by the grace of His Mercy.
The reason I find this so fascinating is that it explores one of the central historic tensions in Shi’ism that is very much around today. On the one hand, it is quite important to render the very notion of the Imamate central to Shi’i theology–that after the Prophet Muhammad there came a sequential group of twelve male lineal descendants known as the Imams, that the Imams have an ability to understand the secret meanings of the Qur’an, that to them is owed the same temporal and spiritual obedience as to the Prophet himself, and that the last of these remains hidden among us and will make himself known one day. This really is the pith and pit of Shi’i theology, what makes it the most distinctive, or to use the phrase of the Fifth Imam, the essence, the climax, the key.
The competing tension, however, is that you do have to be careful not to go too far in the alienation of Sunnis who of course do not share the belief in the Imam. If you render the Imamate as like the Prophethood, in all theological respects, then you are effectively saying that he who denies it is equivalent to one who denies the Prophethood of Muhammad. That at best renders a Sunni a ‘person of the Book’, equivalent to Christians or Jews, and at worst a heretic as a Baha’i is often regarded. It’s not strategically wise for a minority to hold to such a position, and it does, in modernity, tend to cut as against core normative judgments of most Shi’a respecting the status of Sunnis.
So there needs to be a balance, a careful one. Emphasize the absolute equivalence of belief in Imamate to belief in Prophethood, to deny either is an equal theological deviance, then you have a real problem with relations with Sunnis and how to treat Sunnis. Render this whole distinction of little moment, that really we do believe in the same Qur’an, and our substantive Islamic rules are so similar, and we both pray and the like and let’s just call the Shi’a a fifth school of Islamic thought, etc. Sounds nice, but to the committed theologian, all of a sudden, you’ve made belief in the Imamate rather trivial, akin to Shafi’i’s acceptance of some commercial practices shunned by Ahmed, which most Muslims don’t even think about. So what to do?
A few things. First, legally, by rule, you have to declare the Sunni a “Muslim”, as Shi’a jurists do, if there is to be any hope of getting along in this world. So for all temporal purposes (who one can marry, for example, or to or from whom one can inherit) there is no distinction drawn. Again, it has to come out that way. If you start saying that Sunnis cannot inherit from Shi’a, even as Christians cannot inherit from Muslims (I am describing these clear rules, not defending them let’s be clear), then it’s pretty hard to see how anything but permanent hostility will result so long as traditional rules are heeded.
But then, as the passage indicates, you must describe them as somehow deficient in belief, or what is the whole Imamate about? How can belief in the Imamate be so central on the one hand, and not somehow an impediment to true faith on the other? So here it pulls necessarily the other direction. A Muslim, but not a Believer, meaning not one whose faith is really complete, it’s lacking something important.
But then you have to ask if not a believer, then why a Muslim? What is it that renders someone whose faith is missing one key component of what the Shi’i jurist would consider true Islam into a person his daughter can marry when she cannot marry a Christian (again, describing rules, not defending them). And here, of course, we can turn and take refuge in one of God’s greatest and most extolled attributes, Mercy. Ok you’re missing something, something important, something that renders you not one of the “People of Faith”. But God is Merciful, he knows your deeds, he sees you have prayed and fasted and made the pligrimage to Mecca and so because you are good, to Paradise you go. And why on earth couldn’t my child marry a man to heaven bound?
So it works out decently well, or as well as it could under the circumstances.
الكافي ج 2, كتاب الإيمان والكفر, باب دعائم الإسلام, ح 5: علي بن إبراهيم ، عن أبيه وعبد الله بن الصلت جميعا ، عن حماد بن عيسى ، عنحريز بن عبد الله، عن زرارة ،
عن أبي جعفر ( عليه السلام ) … قال : ثم قال (أبو جعفر): ذروة الامر وسنامه ومفتاحه وباب الأشياء ورضا الرحمن الطاعة للإمام بعد معرفته ، إن الله عز وجل يقول : ((من يطع الرسول فقد أطاع الله ومن تولى فما أرسلناك عليهم حفيظا)) أما لو أن رجلا قام ليله وصام نهاره وتصدق بجميع ماله وحج جميع دهره ولم يعرف ولاية ولي الله فيواليه ويكون جميع أعماله بدلالته إليه ، ما كان له على الله عز وجل حق في ثوابه ولا كان من أهل الايمان ،
ثم قال : أولئك المحسن منهم يدخله الله الجنة بفضل رحمته.
 النساء : 80