Sanctification

Forgiveness and the new birth are the start, not the finish. Once a person has turned, been forgiven, and been made alive, an honest question remains: what now? You are accepted, but you are plainly not yet what you will be. Sanctification is the word for that long middle, the lifelong work of being made holy, of being slowly remade into the likeness of Christ. It gets distorted in two opposite directions. Some make it grim self-improvement, white-knuckle willpower. Others make it pure passivity, "let go and let God." The Bible describes something stranger and better than either: a real partnership, in which God Himself does the changing, and we are anything but spectators while He does.

Already holy, and being made holy

Scripture uses the word in two senses at once, and you need both. In one sense you are already sanctified, set apart and counted holy the moment you are in Christ: "ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus" (1 Corinthians 6:11), and "by one offering he hath perfected for ever them that are sanctified" (Hebrews 10:14). And in another sense you are being sanctified, changed by degrees over a whole life: we "are changed into the same image from glory to glory" (2 Corinthians 3:18). A status has been given; a process is underway. The first is the ground of the second. You do not climb toward acceptance; you grow out of one already secured.

The early church saw this growth as built into the very design of humanity, that God made us not finished but to mature toward Him. Irenaeus put it so around AD 180:

The goal is a Person, not a self-improvement project

The aim of all this is not a tidier, more respectable version of you. It is Christ. God's settled purpose for everyone He saves is "to be conformed to the image of his Son" (Romans 8:29), and Paul labors over his churches "until Christ be formed in you" (Galatians 4:19). That is why holiness is not a list of prohibitions but a face. It is the call God gave from the beginning, now spoken to His people again: "Be ye holy; for I am holy" (1 Peter 1:16), and the call Jesus made even sharper, "Be ye therefore perfect, even as your Father which is in heaven is perfect" (Matthew 5:48). The standard is nothing less than the character of God, which is exactly why it can only ever be His gift and His work in us.

It is God's work

Sanctification is not self-help with a religious coat of paint. The One who began it is the One who carries it: "he which hath begun a good work in you will perform it" (Philippians 1:6). When Paul prays for holiness he asks God to do it, "the very God of peace sanctify you wholly," and then promises, "Faithful is he that calleth you, who also will do it" (1 Thessalonians 5:23-24). Jesus Himself prayed for our sanctification and named the instrument, "Sanctify them through thy truth: thy word is truth" (John 17:17). It is God who works the change, by His Spirit, through His word. Whatever effort we make is a response to that, never the engine of it.

And it is our work, too

And yet, in the same breath the New Testament refuses to let us go limp. The most famous statement of the partnership puts both halves a sentence apart: "work out your own salvation with fear and trembling. For it is God which worketh in you both to will and to do of his good pleasure" (Philippians 2:12-13). Not "work for," but "work out": live out, with everything you have, the life He is working in. So the Bible can command, "follow… holiness, without which no man shall see the Lord" (Hebrews 12:14), and "if ye through the Spirit do mortify the deeds of the body, ye shall live" (Romans 8:13). The mistake of moralism is to leave God out and strain; the mistake of passivity is to leave ourselves out and drift. The gospel keeps both in: we work because He works, and our effort is the very place His power shows up.

By the Spirit, not the flesh

Crucially, the change is grown, not manufactured. Paul does not say "produce these traits"; he calls them fruit, what the Spirit grows in a life turned toward Him: "the fruit of the Spirit is love, joy, peace" and the rest (Galatians 5:22-23). And the way forward is less a matter of fighting harder against sin than of walking closer to the Spirit: "Walk in the Spirit, and ye shall not fulfil the lust of the flesh" (Galatians 5:16). Holiness, it turns out, is more like tending a garden than winning a war by sheer force: you put yourself where the growing happens, in the word, in prayer, in the company of God, and the fruit comes (see The Holy Spirit).

Where this lands

Holiness has a forbidding sound, as if it meant becoming less human, more rigid, harder to be around. It means the opposite: becoming, at last, the person you were made to be, free of what deforms you, like Christ, who was the most fully alive human who ever lived. You do not have to carry the project alone, and you do not have to finish it by Friday. God is committed to completing what He started, and He is patient. So the call is not "try harder." It is "abide": stay close to the One who is doing the work, keep turning back when you fall (see repentance), and trust the grace that saved you to also change you (see grace).

Related: Grace, Born Again, Repentance, The Holy Spirit, and The Security of Salvation. Scripture from the King James Version, linked to BibleHub; the Father's word is marked in gold, the Son's in purple.