When I was young, I thought thou was a term of the greatest respect, so formal and lofty it was reserved only for God (and thus should be Thou). As I got a bit older, I realized that it was also used in Shakespeare and other olde thinges, and I inferred from that that it was poetic and formal and lofty. And so its use was for lofty things, including but not limited to God.
And then I learned the truth.
Many people get through their entire lives never learning the truth about thou. There are people who have lived full educated adult lives whom I have met (and even been related to) who insist on using Thou with God and not with anyone else and would tell you quite hotly that it is inappropriate and demeaning and so on to call God You (let alone you).
Which is how misunderstood old things that have lost their original function become signifiers of adherence to tradition and social subordination. (Looking at you, certain grammatical traditions and certain bibliographic practices. Also some items of clothing.)
You know the truth about thou, right? If you’ve learned French or German or Spanish or any of several other Indo-European languages, you will have learned tu or du or a similar word, depending on the language; they all descend from Proto-Indo-European *túh₂, and they all signify the second person singular… familiar. They’re the word you use for people who are close to you and/or are below or at least equal to you in the social order. I like this video on choosing between du and Sie (‘thou’ and ‘you’) in German:
The short of it is that general usage is leaning ever more towards du, but if someone is older than you and/or less familiar and/or a policeman (!), you should use Sie.
And that’s generally the way of it throughout Western Europe. Respect for monarchs and their various hangers-on and other self-important creeps required using the plural, which in English was (and is) you. (It used to be ye and you like I and me or thou and thee, but we stopped using ye and now many of the thou-revering people also believe that ye is a very formal and holy way of addressing People Who Are Definitely Not God.)
In much of Western Europe, this distinction remains a bedevilment and a cause for awkward self-conscious social navigation, but in England they at last cut the Gordian knot and treated everyone like kings and queens. Just about the only people who insisted on using thou with every singular person were the people who refused to dress ostentatiously or do “hat honour” – taking off one’s headwear in the presence of a social superior – or anything else that signified inequality of persons: the Quakers. (Modern-day Quakers – of whom I know a few – are still focused on being humble and down-to-earth, but they use you; thou has become ostentatious.)
Not all Western European societies went with the formal/familiar distinction, I should say. The Celtic languages and cultures have reliably kept the singular form for everyone – in Irish, you can address the head of the government or a priest or even the best footballer in the nation as tú if you’re talking to just the one of them. And the same holds true in Icelandic; it’s not just that the language has changed little over the past millennium, it’s that when the courtly norms of politeness were being established in Western Europe, the newspapers (and pretty much anything else) weren’t being delivered to Iceland. So, again, everyone from lowest to highest, lover to stranger, is þú when addressed individually.
Which, incidentally, is the Old English form – well, þu without the accent. It got respelled due to various orthographic changes and then repronounced due to various vowel shifts, all of which I talk about in a presentation I gave a while ago.
So how did God come to be thou? Well, in the original source texts (Greek and Hebrew, and the secondary Latin), God was addressed using the second-person singular, which was taken as indicating a very close, even intimate relationship (quite the opposite of what many “conservative” Christians envision today, which fuels the confusion). As it happens, that’s also the kind of relationship that the 20th-century philosopher Martin Buber described as ideal not only between human and deity but between any human and any other human – or any other thing at all. He made a distinction between ich–es (I–it) relationships, in which one views the other as an object, separate from oneself, and ich–du (I–thou in translation, not I–you), in which one has an actual personal relationship with the other without hard boundaries, recognizing the other as a potential I (and the other does not have to be a person, either). Effectively ich–es is the way of atomism and ich–du is the way towards monism.
I am definitely in favour of ich–du relations, though I am not especially better than many another in creating and existing in them. But I think this would be a good year to focus on such. After all, it’s 2021, and the 20th letter is tand the 21st letter is u, so 2021 is the year of tu, which means also du and thou and the rest. It’s a year to be casual and friendly and open – to the extent permitted by circumstances, of course. Oh, and then we can keep it up in 2022 and beyond.