Counted Righteous in Christ. The quote is on page 22 of the PDF edition. Dever begins this section of his talk at 32:50 in the video.
Here's the quote from Piper.
To begin with, the older I get, the less impressed I am with flashy successes and enthusiasms that are not truth-based. Everybody knows that with the right personality, the right music, the right location, and the right schedule you can grow a church without anybody really knowing what doctrinal commitments sustain it, if any. Church-planting specialists generally downplay biblical doctrine in the core values of what makes a church “successful.” The long-term effect of this ethos is a weakening of the church that is concealed as long as the crowds are large, the band is loud, the tragedies are few, and persecution is still at the level of preferences.
But more and more this doctrinally-diluted brew of music, drama, life-tips, and marketing seems out of touch with real life in this world—not to mention the next. It tastes like watered-down gruel, not a nourishing meal. It simply isn’t serious enough. It’s too playful and chatty and casual. Its joy just doesn’t feel deep enough or heartbroken or well-rooted. The injustice and persecution and suffering and hellish realities in the world today are so many and so large and so close that I can’t help but think that, deep inside, people are longing for something weighty and massive and rooted and stable and eternal. So it seems to me that the trifling with silly little sketches and breezy welcome-to-the-den styles on Sunday morning are just out of touch with what matters in life.
Of course, it works. Sort of. Because, in the name of felt needs, it resonates with people’s impulse to run from what is most serious and weighty and what makes them most human and what might open the depths of God to their souls. The design is noble. Silliness is a stepping-stone to substance. But it’s an odd path. And evidence is not ample that many are willing to move beyond fun and simplicity. So the price of minimizing truth-based joy and maximizing atmosphere-based comfort is high. More and more, it seems to me, the end might be in view. I doubt that a religious ethos with such a feel of entertainment can really survive as Christian for too many more decades.