I don't know about you, but when I read that verse, there's a lot of tension as I measure it against my own life. The life that I am living most days is not the "new creation" that I would have imagined. Most days, the things I say and do are much more like the old Jared than I would like.
Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come.
(2 Corinthians 5:17 ESV)
So what gives? I want a reboot, an implant. I want an Invasion of the Body Snatchers sort of experience. Or for something more modern, I want to be a totally different person walking around in a Jared suit (a reference from Men In Black).
But I don't think that's what God ever intended. Yes, we receive an alien righteousness in Christ. But when it comes to sanctification and growing "in Christ", it's a lot more messy than it is clean. It's a lot more of a slow gradual process than it is immediate results.
Against the Grain of our Hearts
Imagine running your hand down a splintery 2x4 piece of wood. If you run your hand along the piece of wood with the grain, you're fine. But if you run it against the grain, it's rough and painful as you pick up splinters. May I suggest that our hearts are like that piece of wood?
When we are made a new creation, God takes the grain of our hearts and turns it 180 degrees. We may still try and do the sinful things we did in our old life out of habit and routine, but now there's a pain and resistance as we are behaving in a way that is "against the grain" of our deepest selves.
As new creations, the thing that immediately changes isn't usually our behavior from top to bottom. Instead, what changes is the deepest orientation of our hearts, and the change of behavior can often be a painfully slow process as we learn (literally) by trial, error, and splinters.
When we are saved, behavior that used to be with the grain of our rebellious hearts is now against the grain of our re-oriented hearts. So when we sin—that same old sin—and it grieves us, we can rejoice because we are feeling the splinters of a heart that is a new creation!
Really New vs. Completely New
Now I am not calling Paul a liar. In Christ, at the moment of salvation we are really made new. But we have not yet been made completely new. For that we wait, we groan, and we pray "Come quickly, Lord Jesus!".
So we can say, with confidence, that "In Christ now I have been made really new. With Christ someday I will be made completely new". This give us, as Thomas Chisholm wrote, "strength for today and bright hope for tomorrow".
Come quickly, Lord Jesus!