It's such a long word — sanctification. And it has such a churchy ring to it. No one uses this language outside the church.
So why not adopt a simpler term from the secular world and freight it afresh with Christian content?
While expositing Romans 6:23, John Piper went off script (it's in the audio, but not the manuscript) to tackle this question and also provide a short but substantive definition of what Christian theology means by the term "sanctification."
Now I know sanctification is a church word. I don't think I've ever seen the word sanctification in the Minneapolis StarTribune. It's a church word.
So someone might say, "Why don't you choose a non-church word?" There are reasons for that — not the least of which is that sanctification is built on the Latin word sanctus, which means "holy," and the world doesn't know anything about holiness. That's a church word too.
I will not let the scope of my reality be governed by the paucity of vocabulary in the fallen, secular world. What a folly to think that the world could create enough words to treat infinite realities we learn from the Bible!
The force must go the other way. There are realities in the Word about holiness that the church must teach the world to know — we have to tell the world what it doesn't have a clue about, and then create words for it, and say, "You have to get the word holiness into your vocabulary."
We need the word sanctification. If we tried to built it on a non-Latin word and said "holification," it wouldn't be any better. Where there are words with enough overlap, I'm happy to take them — plunder the Egyptians — and use what they have, and make it plain as I can. But I'm not going to sacrifice the word sanctification because nobody in the world uses it. I'm going to teach it.
So here's the definition: progressively becoming like Jesus. Gradually becoming like Jesus, or becoming holy. Becoming conformed to the image of Christ. Little by little, over time — from conversion till Jesus comes back, or you die — you are in the process of sanctification, becoming sanctified, becoming holy.
For a powerful primer on sanctification, read or listen to Piper's 2000 Christmas Eve sermon on Romans 6:23, called "The Free Gift of God Is Eternal Life."