From the desiring god website:

Part 1: Earlier this Spring John Piper and Tim Keller sat down to discuss the biblical vision of sanctification. In this 14-minute video they touch on how justification and sanctification relate, along with the psychological dynamics of faith.

Time-markers:

00:20 — How Keller talks about sanctification.

2:25 — What are we conforming to in sanctification?

3:45 — How does justification relate to sanctification?

6:40 — The psychological dynamics of faith.

9:00 — What does it mean to “owe God everything”?

11:20 — “I’m going to work my tail off for Jesus, and it’s all of grace.”

Part 2: The discussion continues in this 15-minute video, including some practical guidance for in-the-moment motivations for holiness.

Time-markers:

0:01 — When action is enticed by a blessing.

3:00 — Looking for the past and future for sanctification now.

4:44 — How pleasure relates to our motives.

7:10 — The battle against pornography and being a “John Owen guy.”

11:10 — Three levels of motivation for holiness.

11:29 — Strategies and means of grace for holiness.

Tim Challies conducted a short interview with Dr. Piper. He writes:

A few weeks ago I had the opportunity to interview John Piper. I promptly solicited questions from you, the readers of this site, and Pastor John was kind enough to answer them. Because the focus of this year’s Desiring God National Conference is sanctification, I asked him questions related to that subject. In this interview he discusses why sanctification is not an instantaneous act, how we can emphasize personal toil in holiness without diminishing the goodness and sovereignty of God, why we need to continue to confess our sins to God, and how we can know if we are growing in sanctification. If you read only one of the answers, be sure it is the final one!

What is God’s purpose in making sanctification a lifelong pursuit rather than an instantaneous act at the moment of conversion?

First, I agree with the assumption that this is true. God does do this. That is, he intentionally does not conquer all our sins in an instant, though he could. He could perfect us now. We know this because he is going to do it when we die. We will not sin in heaven. We will be among “the spirits of the righteous made perfect” (Heb. 12:23).

And we know that God will finally throw Satan into the lake of fire (Rev. 20:10) and take away his influence in the new world entirely. If he will do it then, he could do it now. But he doesn’t. He gives Satan leash. So why is Satan allowed to rage, and why does God let us go on stumbling toward holiness?

I am not aware of any text in the Bible that answers this question explicitly. So we answer with inferences from God’s broader statements of purpose. The largest answer is that God does all things for the greater display of his glory, and so this too must be for his glory.

One clue to make this more specific comes from Romans 9:22-23. Paul asks rhetorically, “What if God, desiring to show his wrath and to make known his power, has endured with much patience vessels of wrath prepared for destruction, in order to make known the riches of his glory for vessels of mercy?”

Could God’s purpose in “enduring vessels of wrath” be a pointer to his enduring the sins of his people and the raging of his arch-enemy?

In the case of those who will never repent, God’s patience intensifies the display of his power and wrath. And when God forgives his people 70 times 7 times 7 times 7, does he not intensify the display of his mercy?

And does he not provide the most intense experiences of our own humbling and remorse as we see what becomes of us when we fail to trust God’s grace and power? And so God displays both our inadequacy and his all-merciful sufficiency in allowing us to go on stumbling toward holiness.

How do you emphasize personal toil and effort in sanctification without diminishing sanctification as an act of trust in God?

First by seeking to maintain the biblical proportions. The barometer of our balance is the Bible, not the sentiments of our audience. Over the long haul are we speaking in biblical proportions? This requires not just adding up the effort-passages and the rest-passages, but also being so immersed in the spirit of the Bible that we discern the spiritual tone of how to speak of both.

We do not gloss over the words of Jesus, “Strive to enter through the narrow door.” (Luke 13:24)

Or the words of Paul, “Every athlete exercises self-control in all things. They do it to receive a perishable wreath, but we an imperishable. So I do not run aimlessly; I do not box as one beating the air. But I discipline my body and keep it under control, lest after preaching to others I myself should be disqualified.” (1 Cor. 9:25-27)

Or the words of Peter, “Be all the more diligent to confirm your calling and election” (2 Peter 1:10).

But we bring these commands under the blood-bought promise of God’s commitment to complete the work he has begun (Phil. 1:6) and work in us what pleases him (Heb. 13:20-21) and sanctify us wholly (1 Thess. 5:23-24), and “fulfill every resolve for good and every work of faith by his power” (2 Thess. 1:11).

We never forget that there is a “holiness without which no one will see the Lord” (Heb. 12:14). And we never for get that “by a single offering he has perfected for all time those who are being sanctified” (Heb. 10:14). Because he strove with sin and death and Satan, and triumphed, we strive with the assurance we too will prevail.

Why do believers need to continually confess their sins to receive forgiveness for them? If Christ has paid for sins past, present, and future, what actually transacts when I confess my sin to God?

One might think that this problem is created by the contrast between John and Jesus on the one side, and Paul on the other. 1 John 1:9 says, “If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.” Similarly Jesus tells us to pray, “Forgive us our debts,” with the same frequency as “Give us this day our daily bread” (Matt. 6:12).

But Colossians 2:13-14 says, “You, who were dead in your trespasses and the uncircumcision of your flesh, God made alive together with him, having forgiven us all our trespasses, by canceling the record of debt that stood against us with its legal demands. This he set aside, nailing it to the cross.” And Ephesians 4:32 says, “Forgive one another, as God in Christ forgave you.” (See also Col. 3:13.)

So Paul seems to treat forgiveness as past and secure, while John and Jesus seem to treat it as future and contingent—“If we confess . . .”

However the problem is not between John and Paul, or Jesus and Paul, but between John and John. 1 John 2:12 says, “I am writing to you, little children, because your sins are forgiven for his name’s sake.” Here John speaks like Paul. All your sins have been forgiven (perfect tense). So the tension is between 1 John 1:9 and 2:12. This matters because while one might be tempted to pit author against author, it is harder to pit an author against himself.

I think there are two kinds of solution.

1. The solution in 1 John comes from noticing what John says in 1:7-8, “If we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin. If we say we have no sin, we deceive ourselves, and the truth is not in us. If we confess …” Confessing our sins is the opposite of “saying we have no sin.” If we say that, “the truth is not in us.” That is, we are not Christians. We are not born again.

So John sees “confessing our sins” as a habitual way of seeing ourselves “in the light.” Walking in the light (v. 7) includes not saying we are sinless. Walking in the light means seeing sin for what it is and acknowledging it as such. If we walk in the light this way, that is live a life of acknowledged (rather than denied) sin, “the blood of Jesus cleanses us from all sin.”

All sin! In other words, John is not thinking that every sin must be recalled and confessed, else it’s not forgiven. Rather, he is thinking: Are we real Christians? Or are we deceiving ourselves by denying we have sin? If we are real, if we are walking in the light, if we have a continual acknowledgement of our sin, then we are truly born again, truly connected to Christ, truly under the cleansing power of the blood of Jesus.

In short, John 1:9 is saying that, if we confess our sins, the truth is in us, and we are really born of God and united to Christ so that all the forgiveness that Christ purchased is ours.

2. When Jesus teaches us to pray daily, “Forgive us our debts …” he is probably pointing us in the same direction as 1 John (and Paul). We need not take him to mean that every sin must be remembered. He knew the psalmists cry, “Declare me innocent from hidden faults” (19:12). We do not know all our sins. We can’t confess them all by name.

So Jesus probably means: Be aware of your sinfulness. Take note of your sins. Feel their sting. Be grieved by them. Do not hide them. Bring them before God, and ask that the Messiah’s blood cover them all. Jesus knew the plan of Isaiah 53:6, “The LORD has laid on him the iniquity of us all.” And he knew he was the one who would bear the sins of many. “The Son of Man came to give his life as a ransom for many” (Mark 10:45).

So when he told us to ask daily for forgiveness he meant, Ask that God daily apply to us what Christ bought for us. The price of forgiveness was paid once for all. The application to particular sins in our lives is experienced day by day. We add nothing to the purchase. “Father, forgive us,” does not contradict, “The Lord has forgiven you” (Col 3:13). It means: Apply that decisive purchase, that decisive letting go of sin to this days shortcomings.

How can you know if you are making progress in your sanctification and how can you know how much progress you are making? Is sanctification something that can be measured?

Paul believed that sanctification has degrees. You can grow. He prays that “your love may abound more and more” (Phil. 1:9). He says the Thessalonians are pleasing God and tells them to “do so more and more” (1 Thess. 4:1). He tells the Corinthians that God will “increase the harvest of your righteousness” (2 Cor. 9:10). And prays, “May the Lord make you increase and abound in love for one another and for all” (1 Thess. 3:12).

But the New Testament does not quantify these degrees. “More and more” and “increasing” are discernible but not measurable. That is, while length is quantified in inches and feet. Holiness does not have similar measuring units.

So how do you know you are making progress? There is a paradox here. On the one hand, greater holiness is greater victory over sins. But on the other hand, greater holiness is greater sensitivity to and hatred for sin. So it does not follow that becoming more holy will mean becoming more happy with oneself. You may be a better person tomorrow and feel worse about the smaller corruption that remains.

But there are ways to discern growth. One is that those around you will see it and confirm it. Another is that you will see some of it. You will feel the weakening of some temptations as love for Christ pushes the desirability of sin far away. You will feel drawn to holy acts that once were burdens. And you will have holy sorrow when you omit them, not just guilt over a duty neglected. You will see the preferences in your life change. What was once supremely desirable is trumped by a superior desire for Christ and his word and his way. And you will confirm your heart change in action. Action that becomes less burdensome as love grows. “This is the love of God, that we keep his commandments. And his commandments are not burdensome” (1 John 5:3).

We can and should make progress in sanctification. And we can and should be aware that it is happening. This will encourage us that God is at work in our lives. And it will humble us because progress will mean we can see more clearly how far we have to go and how small are our advances. And how much we will always need a great Savior.