According to the article “you” was formal and “thou” informal.

Yes - and this is how it is in a lot of other Indo-European languages too.

from the spelling i assume they were pronounced the same, but hopefully they both rhymed with thou.

Note that "you" is actually the object form, so the table is unfortunately not arranged the obvious way:

    single  plural  used in contexts of
    thou    ye      nominative (subject), vocative (preceded by the word "O"). For the singular, this implies that the relevant verb takes the "-[e]st" suffix.
    thee    you     all object forms (direct object, indirect object, object of preposition)
    thy     your    possessive determiner, before consonant sounds - thy will, thy God
    thine   your    possessive determiner, before vowel sounds (including 'h') - thine enemy, thine head
    thine   yours   possessive pronoun - not my will but thine, all that I have is thine

(the last distinction used to by used for my/mine, and is still used for a/an)

"In the 17th century, thou fell into disuse in the standard language"

Why would that need to be common knowledge 3-400 years later then?

Because there are many still-popular and important literary works like those of Shakespeare and the King James Bible which benefit from understanding the distinction.

For most of my life I've quietly despised the people in the church of my youth for switching to thee/thou/thy in public prayers. I always thought this was grandstanding performance of piety, or cargo cult mumbo jumbo, or both.

It just seemed so silly to think that God only understands archaic English.

Now it turns out these people I despised were only being respectful, and trying to use the formal forms?

However, since it seems that "thee" and "thou" were actually the informal forms of "you" in 17th century English, now I'm really confused why they suddenly started talking this way the moment they crossed the threshold of the church.

As far as I know, you actually is the formal, originally plural version (ye/you/your) and thou was the informal version (thou/thee/thy/thine). Over time, thou became impolitely informal and is now no longer used, though interestingly enough, nowadays it might even be perceived as more formal than you because it's archaic and survives almost exclusively in liturgical language.

> However, since it seems that "thee" and "thou" are the informal forms of "you", now I'm really confused why they do this.

Unnecessary reverence of the King James Bible translation, probably.

In Old English "thou" is singular, "you" is plural.

As noted in Wikipedia:

> As William Tyndale translated the Bible into English in the early 16th century, he preserved the singular and plural distinctions that he found in his Hebrew and Greek originals. He used thou for the singular and ye for the plural regardless of the relative status of the speaker and the addressee. Tyndale's usage was standard for the period and mirrored that found in the earlier Wycliffe's Bible and the later King James Bible.

Later, presumably due to French influence, "thou" became informal and "you" formal.

Finally "thou" was dropped from everyday speech, though it still shows up in various old phrases.

In ancient Hebrew and ancient Greek, there was no T/V distinction, so their equivalent of "thou" was never considered informal. Some Early Modern translators of the bible tried to preserve the thou/you distinction as it existed in ancient Hebrew and ancient Greek, while deliberately ignoring how it worked in the languages they were translating into. This then influenced the language of prayer.

Another pattern is that religious language tends to be archaic. For example, Jews pray in ancient Hebrew, Muslims in classical Arabic, some Catholics in classical Latin, and so on. I don't know whether this is done to create a sense of awe and mystery, or because religions try to appear unchanging.

Shakespeare, Chaucer, Bronte, Cervantes etc. Unless youre suggesting knowledge of classic literature shouldnt be common?

Or King James Bible - some Christians stick to the original translation (1611).

Why do you think the KJV is the "original translation"? As far as English translations of Scripture go, it was preceded by the Wycliffe Bible. And a translation of the Gospels was produced in the Old English era.

While technically true this is somewhat pedantic, the KJV was the first vernacular translation to be widely available in the English speaking world and is a hugely influential text for literary English, so just take "original" here in a metaphoric sense.

Everybody uses the 1769 update actually.

Not everything is about the only thing that’s been on _your_ mind lately. Other stuff exists, and it doesn’t all have to fit a single narrative.

> But for non-native speaker, this would be mysterious.

As non-native speaker I had learned early that 'you' so I thought that there was probably some other form no longer used.