One of the central themes of StrangerInaStrangeLand.

''So why "thou" and not "you"?''

'' '''Thou''' is explicitly singular, where '''You''' admits a plural. '''Art''' is a conjugation of "to be", emphasizing use as a gerund. ThouArtGod, then, seems a less clumsy way of saying, "you, yourself, personally, here and now, are creating the universe." This, in conjugation with StrangerInaStrangeLand's other mantra, ShareWater, emphasizes a transactional interpretation of godhead, with the universe a kind of shared repast among a myriad creative souls. Reading this way we must say of you as you read this that ThouArtGod.''

'''Art''' ain't a gerund.

'''''God''' is the gerund here. To play on it, art like latin ars - translation work. Not art in heaven, but art right where thou art. See Why's GodIsSomethingYouDo.''

----
Why asketh thou?

''Because archaic language sounds pompous...''

Archaic language has a different set of connotations than everyday language--specifically, it recalls settings where formality and ancient tradition are especially important.  English is actually one of the few modern languages which no longer has an intrinsic means for expressing formality, respect and tradition.

By saying "Thou art God" instead of "You are God", author RobertHeinlein concisely expresses partial agreement with Christianity and partial disagreement.  He has clearly located his branch point using only three words.  Don't you wish that your SourceCodeControlSystem had syntax this expressive?  --BenKovitz
----
Remember also that it is a translation. Heinlein may have wanted it to sound stilted and artificial (rather than pompous) to remind us that the idea cannot be exactly expressed in English. We should be speaking in Martian.

According to the book, if we all learnt Martian we would naturally become right-thinking people. (Cf SapirWhorfHypothesis.)

-----
Then we must not forget, Thou is pronounced (th) as in the, (ou) as in you. to it's Thoo, not Thow. And the a in art should really be pronounced as a shwa. The O in god is tense, not lax (or was when people spoke like this). So it's Thoo urt gode.
----
Furthermore, we should also not forget that ''thou'' was actually
the familiar, ''not'' the formal form of address.  It's interesting
that it now sounds formal, to our modern ears.
-----
And emphatically we must not forget to forget all that. WaitingIs.