Early English translations of the Bible used "thou" and "thee" as the singular second-person pronoun to follow the grammar of the time. But notice in these translations, in the King James version for example, that "thou" and "thee" were not exclusively used for God. Men addressed other men with "thou" and "thee" as well. Like Paul talked to Timothy in 1 Timothy 5:21, just to give an example , "I charge thee before God, and the Lord Jesus Christ, and the elect angels, that thou observe these things without preferring one before another, doing nothing by partiality." Therefore, "thou" and "thee" are mere words and do not produce solemnity nor spirituality.
It is sad for me to notice that many English assemblies these days make a big fuzz about "thou" and "thee" to the point that whenever a brother prays without using these words, he is perceived as carnal or disrespectful. They put a degree of spirituality with these words as if they were solemn and reverential in themselves, when in fact they only mean "you" in today's English. These people remind me of the Messianic Jews who replace English names with Aramaic words in their English Bible translations! They read English Bibles, with of course English words, but they do not accept English names! They say the English names are blasphemous. They do not accept the English name "Jesus" because they say it means "a pig"!!! What a profanity! They mix English and Aramaic in an English translation because for them it is SPIRITUAL. And anybody who does the opposite is carnal or not a 'believer' at all.
In the same way, some assemblies mix modern day language with archaic language to make an "elite spiritual language". Some hundred years ago, "thou" and "thee" were street words. The believers during those years used these words casually as they were part of their colloquial language. But now, the assemblies seem to have made a tradition of "how to address the Lord" by using the special pronouns "thou" and "thee". And the sad part is, they pass it on to younger believers as THE ONLY WAY to talk to God. This practice promotes a wrong sense of spirituality among brethren plus it could confuse visitors who do not know these old English words.
I, personally, do not have anything against those brothers who use "thou" and "thee" when praying during assembly meetings. But I don't think it is right to push this in other brothers-- not letting them use the modern language "you" in praying-- as if it was a direct command or even a doctrine from God.
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"Lord, you know our hearts. Let us glorify you in every word we say and in every thing we do. Amen."