John Piper writes in 50 Reasons Why Jesus Came to Die:
One of the greatest heartaches in the Christian life is the slowness of our change. We hear the summons of God to love him with all our heart and soul and mind and strength (Mark 12:30). But do we ever rise to that totality of affection and devotion? We cry out regularly with the apostle Paul, “Wretched man that I am! Who will deliver me from this body of death?” (Romans 7:24). We groan even as we take fresh resolves: “Not that I have already obtained this or am already perfect, but I press on to make it my own, because Christ Jesus has made me his own” (Philippians 3:12).
Piper continues, referring to this verse. Hebrews 10:14:
“For by a single offering he has perfected for all time those who are being sanctified.”
And so Piper continues:
One of the greatest sources of joy and endurance for the Christian is knowing that in the imperfection of our progress we have already been perfected—and that this is owing to the suffering and death of Christ. “For by a single offering [namely, him-self!] he has perfected for all time those who are being sanctified” (Hebrews 10:14). This is amazing! In the same sentence he says we are “being sanctified” and we are already “perfected.”
There is a tension, a dichotomy, going on here. We are both perfect, and being perfected. It is a mystery of sorts. Both are completely true. In Christ, we are perfect and thanks to Christ we are being made perfect.
It’s as if the princess kissed the toad and he is now a prince. He really is a prince. But now he has to learn to act like that which he really is.
Christians have been made perfect in Christ. Now we just have to learn to act like it.

