This by itself would not be sufficient to avoid gnosticism in the condemnatory clauses; after all, Roman Catholics, Eastern Orthodox, and most Protestants stubbornly and willfully reject atheism, too. (“Most Protestants” do, rimshot. 
) Because of sinful intransigence? No, but not because of ignorance or hereditary error either–not in the case of educated scholars.
Or, let us put the matter closer to home: every Unitarian scholar and student on the planet (whether Christian or Muslim or whatever) stubbornly and willfully rejects the trinitarian faith statement of the AthCreed (not to say the trinitarian implications of the Nicean, etc.) Because of ignorance or hereditary error? Maybe in a few cases, but not likely in the majority.
Because they secretly know it’s true but just don’t want to accept the truth? Obviously they’re sinning in that case whatever the topic might be, and persistently so. In fact, they’d still be sinning even if they happened to be wrong about secretly knowing it’s true!–not because the trinitarian faith is so awesome that it is salvific even if false (a ridiculous idea), but because there can be no good in intransigently refusing whatever light one honestly thinks one sees. It’s a horrible habit of mind to inculcate.
Unless I had some major theological rationales for supposing otherwise, though (which I don’t), I have no grounds for supposing that a majority of unitarian scholars (ones who have studied and are in some position to overcome ‘hereditary errors’–and who think they are doing so, often enough, by ‘overcoming’ the ‘hereditary error’ of the trinitarianism they were raised with!) are secretly having to acknowledge trinitarianism is true but are so hatefully opposed to it that they refuse to do justice to the truth.
Who does that leave over? However-many unitarian scholars (a large proportion of them, even a large majority, possibly near all of them) who are just flat making a mistake–if trinitarian theism is true. (As I find and believe it is, but I am speaking hypothetically for purposes of illustrating the principle.) Or more than one mistake, as may be.
Insofar as a congregation expects God to hopelessly damn those people for being logically inept and/or for having accidentally gotten some data incorrect, then the result is still gnosticism. It may be negative gnosticism, in the sense that it is primarily about condemnation and not about salvation (unlike the actual language of the AthCreed which is at least half about positive salvation!–and which is entirely phrased in terms of positive belief, not about denial.) But it is still gnosticism.
Now: God knows, I would hope that congregations don’t still treat the condemnatory clauses that way. But I know for a fact that some Protestant congregations do treat the condemnatory clauses that way (in principle if not perhaps in fact, not being terribly fond of ‘creeds’ themselves. But they would expect God to condemn atheists or Muslims who have died as atheists and Muslims.) And RCC and EOx congregations, under the leadership of bishops, as well as Anglican congregations, have also in the past treated the condemnatory clauses that way. There was a time, to give one example, when RCCs and EOx basically knew God was going to hopelessly damn the people on the other side, and taught their congregations to expect the same thing, largely over whether someone did or did not accept the filioque. And that, not incidentally, is the kind of culture from which the so-called AthCreed most likely dates from, too, as received.
Also not incidentally, the language of the Quicumque (the traditional title of the AthCreed, from its first Latin syllables) is not “whoever wishes to be saved let him not deny this stubbornly and willfully” (or in any other way) but “whoever wishes to be saved must above all hold this”; and unless he preserves this completely and inviolate he shall without a doubt perish in eternity; if he wishes to be saved, let him think thus about the Holy Trinity; and it is necessary for eternal salvation that he believe rightly about the Lord Jesus; and unless anyone firmly and faithfully believes this he cannot be saved.
This is not language addressed to scholars only, but to everyone. And there is absolutely no leeway in the language for ignorance, or hereditary error (much less for honest mistake). In effect, attempts at reading around the language are at least partial repudiations of the gnosticism of the language. It’s certainly merciful as far as it goes, but (by tautology) unless it goes the distance and distinguishes between honest mistake and willful intransigence against actually perceived truth, then it yet retains the gnosticism by proportion.
more commonly known as Common Era among scholars, but it’s only ‘common’ by being ‘Christian’); and we can gauge from the characteristics of the texts that the authors had at least some concern with being historically accurate. And even though there’s dispute among liberal, moderate and conservative scholars (broadly speaking) about how successful the authors were or the extent to which they were concerned with being historically accurate, the fact that we can have any dispute at all on this topic is because the texts do have characteristics to argue about on this topic.
Or on truly loving other people, for that matter.) The possible pantheistic touches are often found in Gnostic texts, though very much more developed in other texts. Jesus is authoritative but not in any finally special way; more like an especially enlightened teacher. (Not in any finally special way in more flagrantly Gnostic texts either, but far more supernaturally there.) The rejection of women as women, unless they can become men, is sometimes adduced as a parallel to Gnostic texts.