。
I firmly believe in his innocence.
Literal
I [topic-は] his [possessive-の] innocence [object-を] firmly believe-[state-ている].
無実 ('innocence,' specifically of a legal charge, literally 'no substance' — the charge has no basis). 堅く is the adverbial form of 堅い ('firm, solid') — used here for firmness of belief. 信じる ('to believe') + ている for the ongoing state of belief.