John Piper preached on this topic in 1984:
Why do I insist that you must go hard after God, or, which is the same thing, why must we go hard after Christ? Here are four reasons:
1. In Order to Know Him
First, we must go hard after Christ in order to know him. Philippians 3:7–8: "Whatever gain I had, I counted as loss for the sake of Christ. Indeed, I count everything as loss because of the surpassing worth of knowing Christ Jesus my Lord." Paul went hard after Christ, forsaking all the things people normally boast about; and he did it in order to know him.
Why? Because knowing Christ is a value that surpasses everything else. The evidence of conversion is whether you become a Christian Hedonist. Christian Hedonists always go hard after the highest value. They sell everything joyfully for the buried treasure and pearl of great price (Matthew 13:44–45). We must go hard after Christ, because not to means that we don't want to know him. And not to want to know Christ is an insult to his value and a sign of spiritual stupor or deadness in us. But when you go hard after Christ, to know him, the reward is your joy and his honor.
2. To Confirm Our Justification
Second, we must go hard after Christ to confirm our justification. Justification refers to the wonderful act of God in which he forgives all our sins and imputes to us his own righteousness through our faith in Christ. Philippians 3:8–9, "For his sake I have suffered the loss of all things and count them as rubbish in order that I may gain Christ, and be found in him, not having a righteousness of my own that comes from the law, but that which comes through faith in Christ, the righteousness from God that depends on faith."
Philippians 3:9 is clear: the righteousness Paul pursues is based on faith. But he is pursuing it! As a Christian he counts all things as loss in order to have this righteousness. The faith which justifies is a faith which forsakes earthly values and pursues Christ. If justification depends on faith, and if forsaking the world as rubbish is necessary for having the benefits of justification, then it is plain: saving faith is not merely a one-time decision for Christ, but is an ongoing preference for Christ over all other values. The pursuit of Christ is the evidence of genuine faith in Christ as our treasure. Therefore, we must go hard after Christ in order to confirm our justification.
3. Because We Are So Imperfect
We must go hard after Christ because we are so imperfect. Philippians 3:12, "Not that I have already obtained or am already perfect; but I press on to make it my own." We must go hard after Christ because we are so deficient. A failing student should pursue a special tutor. Nearsighted people should pursue an optometrist. People with strep throat should take antibiotics. Alcoholics should pursue a support group. Young apprentices should follow their master at his work.
Not to go hard after Christ means that either you don't trust his power and willingness to change your imperfections, or that you want to cling to your imperfections. In either case, Christ is scorned and we are lost.
4. Because He Has Made Us His Own
The final reason why we must go hard after Christ is that he has gone hard after us and, indeed, has by faith made us his own. Philippians 3:12 again: "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." This sentence explodes the false logic which says that if Christ has found us, we need no more seek him. If he has laid hold of us, we need not press on to lay hold of him.
Paul reasons exactly opposite to this: I press on in order to gain Christ, because Christ has already gained me. Paul's conversion was not a cage to hold him back but a catapult into the pursuit of holiness. The irresistible grace of Christ overcoming Paul's rebellion and saving him from sin did not make Paul passive, it made him powerful!~ John Piper, adapted from, "Going Hard After the Holy God" (sermon preached on January 8, 1984)