pre-noun adjectival
innocent; guiltless; sinless
Pre-noun adjectival (rentaishi) meaning 'without guilt' or 'free from sin'. Often used in literary or formal contexts, and can carry a poetic or slightly archaic tone.
罪なき人々が戦争で苦しんでいる。
Innocent people are suffering because of the war.
彼は罪なき子供のように笑った。
He laughed like an innocent child.
無罪 is a noun meaning 'innocence' or 'not guilty' in a legal sense, while 罪なき is a pre-noun adjectival with a more literary or moral nuance.
無実 is a noun/adjectival noun meaning 'innocent' (especially of a crime), often used in legal contexts. 罪なき is more poetic and less common in everyday speech.
Compound of 罪 (tsumi, 'sin, crime') + なき (naki), the classical attributive form of the adjective なし (nashi, 'not exist'). The modern equivalent would be 罪のない (tsumi no nai).