Esō means within or inside — the interior dimension. It appears in John 20:26 ('eight days later... the disciples were inside [esō]'), Acts 5:23 ('guards standing at the doors, but when we opened them, we found no one inside'), and most significantly in the Pauline description of the inner person: 'I pray that out of his glorious riches he may strengthen you with power through his Spirit in your inner [esō] being' (Eph 3:16). The 'inner man' (esō anthrōpos) is a key Pauline concept for the renewed spiritual self.
Paul's contrast between the outer and inner person (esō anthrōpos) in 2 Corinthians 4:16 and Ephesians 3:16-17 defines the Christian life as fundamentally a work of interior transformation: 'Though outwardly we are wasting away, yet inwardly [esō] we are being renewed day by day.' The Spirit's dwelling place is the interior self — the esō — not merely outward religious behavior. This inwardness is the fulfillment of Jeremiah's new covenant promise: 'I will put my law in their minds and write it on their hearts' (Jer 31:33). True religion moves from the outside in — and then back outward through a transformed life.