坚信不疑 is a four-character set phrase but not a classical 成语. It's a modern set expression formed from 坚信 (firmly believe) + 不疑 (without doubt), used in the same syntactic frame as a chengyu. Pattern: 对 X 坚信不疑.
mistakes
Don't confuse with the rough opposite 半信半疑 (half-believing / half-doubting). 坚信不疑 = total belief; 半信半疑 = uncertain.
