2 Corinthians 3:18 and we all, with unvailed face, the glory of the Lord beholding in a mirror, to the same image
are being transformed, from glory to glory, even as by the Spirit of the Lord. [YLT]
The New Creation is that completed transformation we see spoken about above.
What confuses us is what was said, here in the KJV and many other translations:
2 Corinthians 5:17 "Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new." [KJV]
But notice it here in the BBE:
2 Corinthians 5:17 So if any man is in Christ, he is in a new world: the old things have come to an end; they have truly become new. [BBE]
The word translated as "creature", is G2937 - ktisis - from 2936; original formation (properly, the act; by implication, the thing, literally or figuratively): KJV -- building, creation, creature, ordinance.
From G2936 - ktizo - probably akin to 2932 (through the idea of proprietorship of the manufacturer); to fabricate, i.e. found (form originally): KJV -- create, Creator, make.
So the better translation in that text is, "new creation." And there are more than a few Bible translations which do translate it, "new creation."
That verse is easy to draw the wrong conclusion from as it is actually speaking of the old being gone that the work of the creating is now able to be taking place.
It is a continuous action which has a beginning and works toward an end goal of completeness.
We must live as best we can like anything we desire to be else we never perfect it in us. That is true of being successful at anything we choose to be in life. If I desire to be a well seasoned skilled carpenter I have to live as a carpenter for some time to reach that goal.
Similarly, that new creation. We must begin to live as that new creation or we will never perfect it. And what a waste of God's spirit which he generously gives to help us that would be.
Edit: Let me clarify. Christ is the actual new creation for now in that he is our replacement for Adam. As all was being done in Adam, now all is being done in Christ. He is the primary new creation. We are to be about "putting on Christ." That makes us like the apprentice carpenter. Not a full fledged carpenter as yet but aspiring to become a full fledged carpenter. We are a part of and aspiring to be completed as that new creation in Christ.