My friend Marthame said in a recent sermon that art, as in “who art in Heaven”, is an informal or familiar version of “are”. That is, he said, the translators of the King James Version of the Bible were picking that verb form specifically to instruct us to pray to God as we would speak to a member of our own family.
I’m skeptical about “who art” being an informal/familiar form of “who is”. Booth (1831) quotes Jonson (1640) as saying “I am / Thou art / He is / We are / Ye are / They are”.In other words, in the English spoken in London in the early 1600s—roughly the time of the KJV—the plural form was “are” and the singular was “am / art / is”.
As for 20th-century Quakers using archaic verbs, a letter to the Saturday Review of Literature (1928) says: “One never hears ‘Dost thou?’ or ‘Wilt thou?’ or ‘Thou shalt not’ but, instead, ‘Does thee?’ ‘Will thee?’ and ‘Thee shall not.’ (http://goo.gl/N7kSt)
It’s true that the KJV has two words that modern standard English would render as “you”, but the distinction seems to be grammatical number. εσύ is “thou” (“you”) and εσείς is “ye” (“y’all”).
An excellent example of this is available in Matthew 21:16:
And said unto him, Hearest thou what these say? And Jesus saith unto them, Yea; have ye never read, Out of the mouth of babes and sucklings thou hast perfected praise?
Him is singular, so the priests and scribes said “Hearest THOU?”. Jesus was answering them (plural), so He said “Have YE never read…?”.
All of which is to say: The KJV translators depicted Jesus as instructing us to pray in an ordinary, simple way, not an ornate or affected way. These days, saying “who art in Heaven” is precisely the formulaic, affected sort of speech Jesus wanted us to avoid—and we repeat it in front of each other, when just a few breaths ago Jesus had said, “When thou prayest, enter into thy closet” [in private, not standing in the synagogues] and “use not vain repetitions”.
Our bad, Dude.