Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.
Therefore. Connected with 2 Corinthians 5:16, 'We know Christ no more after the flesh.' As Christ has entered on His new heavenly life by His resurrection, so all who are "in Christ" (i:e., by faith, as the branch is IN the vine) are new creatures (Romans 6:5-11). "New" х kainee (G2537)] implies a new nature quite different from any previously existing: not merely recent [nea], a different Greek word (Galatians 6:15).
Creature, х ktisis (G2937)] - 'creation,' and so the creature resulting from it (cf. John 3:3; John 3:5; Ephesians 2:10; Ephesians 4:23; Colossians 3:10-11). As we are "in Christ," so "God was in Christ" (2 Corinthians 5:19): hence, He is mediator between God and us.
Old things - selfish, carnal views (cf. 2 Corinthians 5:16) of ourselves, of others, and of Christ.
Passed away - spontaneously, like the snow of early spring before the advancing sun.
Behold (Isaiah 43:19; Isaiah 65:17).