Why Do We Need “Thee” and “Thou”?
In almost every language but Modern English, people knew whether the speaker was addressing one person or many. In Classical English, this is preserved. If the speaker is talking to one person, he uses “thee” or “thou.” If he is talking to many people, he says “you” or “your.” The King James preserves this distinction.
When Jesus spoke to Nicodemus, He said, “Marvel not that I said unto thee, ye must be born again” (John 3:7). What He said in effect was, “Marvel not that I said unto thee (Nicodemus), ye (all of you people) must be born again.” Jesus did not tell this only to Nicodemus.
He spoke to all people of all times, from those who stood around that night, even to us today: Ye must be born again!
http://www.chick.com/bc/2000/theeandthou.asp
Second-person pronoun distinction:
Perhaps the first thing that many people identify as an archaism in the KJV is the use of “thee’s” and “thou’s.” The KJV uses these pronouns in order to distinguish between the second-person singular (thou, thee, thy, thine) and the second-person plural (ye, you, your, yours). The Greek and Hebrew make this distinction.
This distinction is crucial for a close reading of the Bible. See Galatians 6:1 for example: “Brethren, if a man be overtaken in a fault, ye which are spiritual, restore such an one in the spirit of meekness; considering thyself, lest thou also be tempted.” By paying close attention to the pronouns, we see that the restoration of a faulted individual is the responsibility of not just one person but of many (“ye” which are spiritual) but each individual must examine his own integrity (considering “thyself”). We cannot extract these helpful teachings on communal responsibility and individual responsibility from this passage unless the distinctions in pronouns are translated. Other passages where the distinction in a pronoun’s person is important are Exodus 4:15, Exodus 29:42, 2 Samuel 7:23, Matthew 26:64, Luke 22:31-32, John 3:7, 1 Corinthians 8:9-12, 2 Timothy 4:22, Titus 3:15, Philemon 21-25, to name a few.
Many modern languages such as French, German, Spanish, Japanese, and Chinese retain this distinction between singular and plural second person pronouns. Moreover, although “thou” and “ye” may be archaic, they are not unfamiliar. We do not use these pronouns in colloquial speech, but we still use them when we sing hymns (even contemporary praise songs) to God and to the congregation. We sing “Be thou my vision” in the song titled thus, and “Prepare ye the way of the Lord” in the song, “Days of Elijah.” At every hockey game Canadians sing their national anthem, “O Canada,” which mentions “thy” once and “thee” four times in one stanza. The song became the official Canadian national anthem as recently as in July 1, 1980 and remains a national favorite. Canadian children do not have any problem understanding the meaning of “thy” and “thee.” Although “thou” and “ye” may not be parts of colloquial speech, they are certainly not obsolete if we still use them in songs, prose, or the Bible when there are good grammatical, metrical, or stylistic reasons to use them.
http://www.kjvtoday.com/home/language-of-the-kjv#TOC-Things-that-are-not-mere-archaisms
Also, don’t forget “My country tis of thee…”