Greek ho esō anthrōpos — "the man within, the inward person." Paul's term for the Spirit-indwelt core of the regenerate believer — the true self that lives to God, distinguished from the "outer man" (the body in its weakness) and from the "old self" (the corrupt nature crucified with Christ). Appears three times: Romans 7:22, 2 Corinthians 4:16, Ephesians 3:16. The inner man is not a second soul; it is the regenerate self in its Godward orientation.
Three passages shape the doctrine. (1) Romans 7:22 — "I delight in the law of God, in my inner being" — even in the midst of the wrestling with indwelling sin, the regenerate person has a Godward inner core that loves the law. This is the sign of true regeneration. (2) 2 Corinthians 4:16 — "Though our outer self is wasting away, our inner self is being renewed day by day." Aging bodies, chronic pain, persecution-battered flesh — none of it can touch the inner man, which is being daily renewed by the Spirit. The 85-year-old dying saint may be younger on the inside than the 25-year-old fitness influencer. (3) Ephesians 3:16 — Paul prays that believers would be "strengthened with power through his Spirit in your inner being." Christ dwells "in your hearts through faith" — the inner man is where Christ makes His home. Implication: cultivate the inner life. Outer success without inner life is hollow. Inner life without outer expression is sentimentalism. But of the two, the inner is primary — it survives the grave, grows forever, and determines your eternal weight of glory. Feed it with prayer, Scripture, silence, fellowship, and the sacraments.