And I wonder if Latin has a "thee" form, since grammatically-historically, Augustine was using Latin and we have to ask what the Latin would have meant to him.
My professor of Classical Latin taught me that the Latin second-person pronoun
tu had no connotation of either politeness or casual speech. It was simply a matter of number. As medieval Latin developed, the second-person plural form
vos came to be used to address individuals, and carried a strong sense of politeness (ie. "
Pax vobiscum.") This trait survives in modern Romance languages (cf. French
vous or Castilian Spanish
vosotros).
That
tu would be the Latin equivalent of the early modern English
thou, or of the modern English singular
you. Latin
Vos, meanwhile, is the equivalent of the English
you (plural in early modern English, and both singular and plural in modern English).
I'm not sure if this helps with anything, but there you go.
I have no idea why the English
you would be considered irreverent, or necessarily plural. But never mind.