As the title states, I am seeking a secular phrase synonymous to "Heaven only knows" or "God only knows." Any ideas or suggestions would be appreciated.
"Who knows?" is the simplest form. I hear it (and use it) regularly.
This one involves swearing, but this is a fairly common way to express the same idea:
"Fuck only knows"
The literal intended meaning of this phrase is "nobody knows". As an atheist, I don't believe there is a God. Therefore, I can still say "God only knows" because the intended meaning of "nobody knows" logically follows from these premises.
If you are from 1950s England, you could say
goodness only knows.
How about "It's anyone's guess"?
Secular phrase you seek. Quote Yoda you can.
Difficult to see. Always in motion is the future..
A secular phrase to the idiom "God only knows"
- It's impossible to say. - "God only knows how he managed to find out where I lived." http://dictionary.cambridge.org/us/dictionary/american-english/god-only-knows
Another choice woule be
- "Nobody knows and neither do I". - "God only knows where I've left my keys".
Most people I know are atheists and we happily use "God knows" and "Oh my god", without reverence or irony. Like all language, it's just a term that has universal currency - the archaic reason for why it exists is really just interesting more than important. In the unlikely event that the world reverts back to a place where it's really important that we constantly demonstrate our atheism, maybe a new term will arise.
A humorous, colloquial version playfully ascribes omniscience to a person:
Only Chuck Norris knows.
In Stack Overflow culture, this would be:
Only Jon Skeet knows.
If you want something that is truly not a minced version, the only thing I can really think of is:
It's a mystery to me.
Or for a cheerier feel, "It's a mystery to me!"
Famous example usage:
(If you can sit through the 1'59" musical intro before the singing starts - remember the 80s?!)
A reason for saying that “God only knows” is that it would be impious or irreverent to suggest that He (who knows everything) does not know.
For that reason, you get expressions like this one on the Tomb of the Unknown Soldier.
A corresponding secular phrase could be simply, "I don't know", "No-one knows", "It's unknown", or "It's a mystery".
How about fate only knows. Since the religious version implies that the future is predetermined and known. (Fate is still a Greek religious concept)