Being a new creation in Christ means that the old sinful person you once were has gone as a result of the finished work of Christ.
We begin a lifelong journey of being renewed and sanctified by the work of the Holy Spirit. God’s purpose is that His people become like the Lord Jesus Christ; therefore the Holy Spirit works in the changed heart and mind of the believer helping them to become conformed to the image of Jesus—to become like Jesus in thought, word, and deed. A new creation indeed!!!
The new creation is a wondrous thing, formed in the mind of God and created by His power and for His glory.