From a 2006 sermon by John Piper on Romans 15:18-24, asking the kids in the congregationif they--like Paul--have a "holy ambition":
Do you have one? Probably not yet. You’re only a child. That’s what you’re supposed to be. But some day you won’t be a child any more. And one of the differences between being a child and growing up is that growing up as a Christian means you get a holy ambition. Most little girls, my Talitha included, really want to have and play with dolls. That’s a good thing. But the day is going to come, little girls, when you will put away the fun of playing with dolls and grow up into the even bigger, better joy of caring for real babies in the nursery. And maybe you will even lead a ministry someday of caring for hungry babies far away, or lonely babies who have no mommy or daddy. And for some of you this will become a holy ambition. For others your holy ambition will be something else.Dave Harvey has been exploring this issue in some depth. See his sermon (and Tony Reinke's notes on it), "Don't Waste Your Ambition."
And boys, listen. If you are like I was, what you really want is a ball, a truck, and gun and somebody to play with. I’ve never had real gun (except a pellet rifle). But I shot a lot of bad guys with my Matt Dillon pistol and my Lukas McCain-like, circle-handled rifle. I loved playing football with my friends and digging roads across the street for my trucks and drawing my pistol so fast you couldn’t see it. It was fun. And that was good.
But some day you won’t be a little boy any more. And one of the differences between being a little boy and growing up is that growing up as a Christian means you get a holy ambition. And that means the fun of guns and trucks and balls gets small and the joy of fighting for justice and salvation gets big. Growing up means getting a holy ambition to wield the sword of the Spirit mightily and drive a truckload of love to the needy and kick Satan’s rear end in the name of Jesus.
Mom and Dad, single people, young and old, Christians should have a holy ambition. Something you really, really want to do for the glory of God. Something that controls you. It helps you decide not to go to Rome yet. It gives eternal focus and passion to your life.
See also his sermon series here: