The Christian is “in Christ” and he/she “is a new creation.”
The rest of the verse says, “The old has passed away; behold (look), the new has come.”
Being a Christian changes your life. The change is not yet perfect (it will be in glory), but the change is real.
Old unbelief has gone, you are no longer dead in your sin, you are no longer separated from God, you are no longer spiritually blind. Faith has come, you are alive, you can draw near to God, your mind is being renewed (Romans 12:2) and you can see (John 9:25).
The previous verse (2 Corinthians 5:16) says, “Even though we once regarded Christ according to the flesh, we regard Him thus no longer.” You are a new creation because of Jesus and that new creation is demonstrated by the fact that you now see Him with spiritual eyes. He is not just a religious teacher, He is the Son of God. He is not just someone who died on the cross, He is your Saviour who died for you.
When we still struggle with the old – the old temptations, doubts and sins – we can encourage ourselves with this: if I see Jesus as He is, the Son of God, my Lord and my Saviour, then I really am a new creation. I do still get things wrong, but I believe in Him, I love Him, my Saviour.
If you love Jesus, your Saviour, you are a new creation. The old has gone and the new has come. You can live today in the assurance of that new life and live that new life for His glory.