“Therefore, if anyone is in Christ, the new creation has come: The old has gone, the new is here!” 2 Corinthians 5:17
“In Christ” can mean “by or through Christ,” as the agent of our salvation. Christ is also the One in whom we find God’s gifts. We are baptized “into Christ” (as Galatians 3:26–27 says, “So in Christ Jesus you are all children of God through faith, for all of you who were baptized into Christ have clothed yourselves with Christ”) and experience what He has.
“New creation” means God has redeemed and restored the believer into a new sphere of existence, re-created as a person of faith in Jesus Christ, who desires to live in and for Him alone.
“Old has gone” means in Christ, the things of the sinful flesh have died. The believer is under the lordship of Christ.