An actual prayer. Six characters, and it's chanted at funerals.

What a Japanese person actually thinks
Devotional and specifically funerary in most people's daily experience. Reads either as sincere religious commitment or, if you have none, as wearing someone else's prayer as decoration.

Meaning

Literally: I take refuge in + Amida Buddha
Actually: The nembutsu — the invocation of Amida Buddha in Pure Land Buddhism. A live, practised prayer, not a decorative phrase.

Scores

Naturalness●●●●●

Correct and universally recognised.

Coolness●●●●●

Six characters is a lot of real estate, and the register is unambiguously religious.

Watch out

mediumThis is a practised prayer of a living tradition. Worn without belief it lands differently than an aesthetic term does.
mediumStrongly associated with funerals and memorial services in everyday Japanese life.

Get this instead

無 mu
nothingness

If Buddhist depth is the appeal rather than devotion specifically, this carries it in one character with no liturgical commitment.

On skin

南無阿弥陀仏

Orientation: vertical

Mincho, vertical, if you keep it.