Is it necessary to believe in Father, Son and Holy Spirit? Or, is it necessary to believe in the official doctrine of the Trinity as secured by ancient councils? I assume those are not identical (I should say, for those who believed prior to the councils). Nonethless, let's assume salvation depends on belief in the doctrine as defined by the councils. If those who preceeded the doctrine did not believe it (because they did not know it) , then are they saved? And if so, how?
It is fundamental to belief in the councils to understand that they were not called to devise 'official doctrine' in scare quotes, but rather to deal with specific heresies that were against
the faith already held by the Church. For sure, over time they
became 'official doctrine' in a way, as (e.g.) the Nicene position won out over that of the Arians (but note how long that took), but if they never had, the doctrines that they formalized would still be what was preached in the orthodox churches. This is how we can read and understand preconciliar fathers such as St. Gregory the Wonderworker, St. Justin Martyr, etc. as still teaching in conformity with the faith, to the extent that they do (NB: St. Justin believed in the preexistence of matter, which is a position that is condemned), without of course their formally having assented to Nicaea, let alone Constantinople or Ephesus (or even later councils, for the Chalcedonians).
If it helps, this is somewhat similar to how we may look at the Biblical canon: Outside of the Christian West (where it was defined largely as a result of/as a reaction to the Protestant reformation, which is uniquely western in its origins and grievances), it took a very long time for the Biblical canon to be settled in the exact form in which the wider Christian world would recognize it. The Book of Revelations/the Apocalypse of St. John (without a doubt the most controversial of the NT books) does not appear in the canon of the Syriac Church, for instance, until 616 AD, in the Syriac version compiled by St. Thomas of Harqel (a.k.a. the Harklean version). Does that mean that centuries upon centuries of Syriac Christians from before 616 AD are condemned for having learned the faith from "incomplete" Bibles? I wouldn't say so, as it's not like the faith therefore didn't exist before 616 AD.
Or perhaps even better, as it concerns the Holy Trinity most directly, the (in)famous Johannine Comma (found in John 5:7-8 in the Latin manuscript tradition, and only in Greek manuscripts starting in the 14th century) is completely absent from the Ethiopic, Syriac, Arabic, and Georgian manuscript traditions (and only present in Armenian manuscripts starting in the 12th century). This statement, and whether or not it is to be viewed as authentic or not, has been the source of a lot of ink spilled over the centuries in the context of debates about the Holy Trinity, particularly between Protestants and Catholics. We should ask ourselves, though, how much it can really matter outside of that specific debate if all of these other traditions, which represent some of the earliest peoples to become Christian in the entire world (Armenia being the first Christian nation, and Georgia the third; the Arabs being mentioned in the scriptures as one of the specific peoples present on the day of Pentecost, etc.) have been able to not just express their Christianity, but specifically their strong and orthodox
Trinitarianism, without having to worry about that statement at all, or perhaps
without even being aware of it in the first place (as again, it has never been present in the majority of them).
Speaking only for myself, I think we can look at questions like yours similarly, as obviously there is a difference between born into the post-conciliar world as we are and living and departing all before 325 AD. It would be foolish to pretend that there wouldn't be, because doing so would essentially treat the councils as though they are optional or of little importance, which is not in keeping with how any form of traditional Christianity believes and behaves. That difference, though, is not to be thought of as a kind of quasi-academic theology quiz, whereby if you don't say the correct phrasing using all the proper words that we now use, you're out. No. But recall from the above examples how it is that people can be completely formed within the bosom of the Church, taking on the mind of the Church, and professing the faith of the Church in any particular era. This extends both into the ancient past and into the far future, as the faith itself is eternal.
To wrap it up, I like to think about questions like this by remembering that my particular church is now and for the majority of its life always has been primarily a church of largely illiterate peasant farmers in Upper Egypt. I don't mean to romanticize being the Egyptian version of a country bumpkin, but it is important to remember for me because otherwise it is far too easy, since I
don't come from that environment myself, to wall myself up in my room with my translations of St. Macarius, St. Shenouda, St. Samuel of Kalamun, etc., etc. and think that by learning a lot of 'stuff', I'm becoming a better Christian. I'm certainly not anti-learning, but it bears repeating that that's not true, or at least not self-evidently so. You can learn a lot or you can learn a little, but what you
experience is what will ultimately drive you. I can't know for sure if the illiterate farmer who goes to his nearest monastery for Holy Week celebrations in Egypt knows the ins and outs of Christological or other Theological debates that have happened throughout all 2000 years of our faith, but I can know that when we pray together
"We believe in One God, God the Father, Creator of heaven and earth...", we are affirming the same faith that we all experience together, and have for 2,000 year
s (even though the Creed itself was obviously not written 2,000 years ago).
