A real Japanese set phrase — but classical grammar and six characters long.

What a Japanese person actually thinks
Slightly grand and old-fashioned, in a good way. Also the title of a well-known manga and anime, which some readers will hit first.

Meaning

Literally: god + only + (emphatic) + knows
Actually: 'Only God knows.' A genuine idiom using the classical ぞ…連体形 emphatic construction, so it has real literary standing.

Scores

Naturalness●●●●●

A real, established phrase. The classical grammar is deliberate, not an error.

Coolness●●●●●

Six characters with three hiragana. Long for a tattoo, and the manga association softens it.

Watch out

low'The World God Only Knows' (神のみぞ知るセカイ) is a popular manga. A slice of readers will think of that first.
mediumIf you meant 'only God can judge me', this isn't that — this is about knowledge, not judgement.

Get this instead

天命 tenmei
the mandate of heaven; one's fate as decreed

Two characters for the same idea of a decision made above your head. Far denser.

On skin

神のみぞ知る

Orientation: vertical

Mincho, vertical.