Earlier quoted context omitted.
> Another example many people outside Brazil find interesting: in my family we were taught to never use the formal towards anyone. The rationale is that everyone is equal and that using the formal language was disrespectful because it created an artificial distance between us and the other person. We were also taught never to use the formal language when praying for the same reason. Interestingly this is why Quakers…
> People used "thou" (the familiar form) in conversations with God. People used "you" as a singular pronoun to be polite. Why didn't people address God with "you"?
Even today, this tends to be the case in European languages that distinguish familiar and polite pronouns (what linguists call the T-V distinction). God tends to be an exception to the usual T-V rules.
The reason for this is that in all these languages, thou started out as simply the singular and you as the plural, with no politeness dimension at all. Using the plural pronoun (or third person pronouns, etc) for politeness was a fad that only spread around Europe in the Middle Ages (give or take).
Religious formulae, however, are generally extremely resistant to language change. This is a very consistent finding across the world; some of our best evidence in historical linguistics comes from religious texts (such as the Rigveda, the Avesta, etc). Religion tends to be, not surprisingly, a highly conservative and ritualised domain.
Thus, prayers in European languages with the T-V distinction generally retain the use of T forms when addressing God. There are all sorts of lovely folk explanations for this, but the real reason is basically just because prayers predate the T-V system altogether.