Popular opinion regards the use of Thee and Thou as a type of formal speech in English, likely because it is archaic and used to refer to God. As it came up recently around here, I just want to review the facts, and hit hard a bit that is sometimes not quite understood even by those who know that those forms are not formal.
Thou is the intimate form, related to and similar to French
Vous Tu and German Du. It is used in reference to God to stress that intimacy. That is considered more important than even the respect that would be communicated by "you." This is surprising enough that it is worthy of some reflection on its own. But the usage is even more surprising. Thou was also used with social inferiors. Thus, it could be used to effect with one's spouses, children, and close friends, and Shakespeare does this in several places, where characters will toggle back and forth between the forms depending on the type of persuasion they are engaging in. The best example is probably the first scene in King Lear (a play that is coming up a lot for some reason), especially the exchange with Cordelia.
Yet even this, the possibility of disrespect in addressing God, was considered a worthy risk to preserve the I-Thou intimacy. We are a fortunate people.