Has either a non-Trinitarian Christian or a non-Christian ever had the truth of the Trinity specially revealed to them, personally, unquestionably, beyond any reasonable doubt, through the Holy Spirit, and then published their testimony about it? I presume testimonies of this kind should be possible in principle, in light of what John 16:13 promises:
When the Spirit of truth comes, he will guide you into all the truth, for he will not speak on his own authority, but whatever he hears he will speak, and he will declare to you the things that are to come. [John 16:13, ESV]
Assuming that the doctrine of the Trinity is true, and since the Holy Spirit will guide us into all truth, it shouldn't be unreasonable to expect to hear about cases of people being led by the Holy Spirit, possibly even in supernatural ways, to accept the truth of the Trinity.
For the purposes of this question, I'm assuming the definition of the Trinity proposed by the Athanasian Creed:
Whosoever will be saved, before all things it is necessary that he hold the catholic faith. Which faith unless every one do keep whole and undefiled, without doubt he shall perish everlastingly. And the catholic faith is this: that we worship one God in Trinity, and Trinity in Unity; neither confounding the Persons, nor dividing the Essence. For there is one Person of the Father; another of the Son; and another of the Holy Ghost. But the Godhead of the Father, of the Son, and of the Holy Ghost, is all one; the Glory equal, the Majesty coeternal. Such as the Father is; such is the Son; and such is the Holy Ghost. The Father uncreated; the Son uncreated; and the Holy Ghost uncreated. The Father unlimited; the Son unlimited; and the Holy Ghost unlimited. The Father eternal; the Son eternal; and the Holy Ghost eternal. And yet they are not three eternals; but one eternal. As also there are not three uncreated; nor three infinites, but one uncreated; and one infinite. So likewise the Father is Almighty; the Son Almighty; and the Holy Ghost Almighty. And yet they are not three Almighties; but one Almighty. So the Father is God; the Son is God; and the Holy Ghost is God. And yet they are not three Gods; but one God. [...]
Appendix - insightful Q&A with @curiousdannii
curiousdannii: The Holy Spirit affirms true doctrine to all Christians. So this is any person's testimony.
OP: Does that include the testimonies of non-Trinitarians?
curiousdannii: Well non-Trinitarians might say that the HS revealed the non-truth of the Trinity to them... Again, I'm going to suggest that you stop chasing after anecdotes. What is this helping you to investigate?
OP: If you epistemologically disregard anecdotes as unworthy of consideration, should I conclude that you also disregard all the anecdotes recorded in the Bible, or do you make an exception for those?
curiousdannii: I don't disregard all anecdotes. But I think they're best told between people in person. And while technically on-topic on this site, they're usually not very interesting or useful. Especially when you ask something which almost every Christian would have experienced, like this one. If someone says yes, what will you learn? If someone says no, what will you learn? Which answer to this question are you expecting? What prompted this question? Is there a doctrine you think implies this should/shouldn't happen? If you contextualised this question it would seem less arbitrary.
OP: Especially when you ask something which almost every Christian would have experienced, like this one - Is that really the case though? I honestly think it's much more likely that most Christians who are Trinitarian believe so because they were indoctrinated by Trinitarian parents, because their first exposure to Christianity was through the lens of a Trinitarian denomination, or any other culturally biased way to become Trinitarian, not because of some "supernatural" revelation through the Holy Spirit. But I can be wrong. (If you think I am, please, go ahead and prove me wrong.)
curiousdannii: Maybe you're thinking about something else than what you wrote. What you wrote was whether the HS reveals the truth of the Trinity to believers, and the witnessing of the HS is a core doctrine of Christianity. I'm not certain if Arminians would agree, but for Reformed Protestants at least no one can be a Christian without the work of the Spirit. It would be a different question if you meant revealed through a vision/dream/something like that. That would be much rarer, sure.
OP: If someone says yes, what will you learn? They would need to provide concrete examples to justify a "yes", and those examples could be further investigated to gain deeper insights. If someone says no, what will you learn? They would have to justify their "no" somehow. If they manage to do it, that would be at odds with John 16:13 (quoted in my question), which would be very, very strange if Trinitarianism is true. Which answer to this question are you expecting? A substantiated "yes"/"no" answer. What prompted this question? Curiosity.
OP: It would be a different question if you meant revealed through a vision/dream/something like that - do you believe it is possible to be a Christian and still believe in doctrines which are false or at least not 100% accurate? If so, how can you know which doctrines a Christian believes are the ones revealed by the Holy Spirit and which ones are not? How can you know for certain that the Holy Spirit has revealed something to you, personally, if not through some remarkably obvious revelatory experience?
curiousdannii: Ah, okay, now I'm starting to understand. That would be a better question to ask, rather than anecdotes which won't get at those epistemological questions.
Especially when you ask something which almost every Christian would have experienced, like this one- Is that really the case? I honestly think it's much more likely that most Christians who are Trinitarian believe so because they were indoctrinated by Trinitarian parents, because their first exposure to Christianity was through the lens of a Trinitarian denomination, or any other culturally biased way to become Trinitarian, not because of some "supernatural" revelation through the Holy Spirit. But I can be wrong. (If you think I am, please, go ahead, prove me wrong.)