In his commentary on 2 Corinthians ( The New American Commentary Volume 29 – 2 Corinthians ), David Garland asks what “in Christ” means in 5:17, and answers:
“This phrase, ‘in Christ,’ can mean several things that are not mutually exclusive: that one belongs to Christ, that one lives in the sphere of Christ’s power, that one is united with Christ, or that one is part of the body of Christ, the believing community. Paul’s assumption is that being in Christ should bring about a radical change in a person’s life.”