But with 40 years of "seriously" studying Scripture under his belt, Warren had no problem discussing the central tenets of Christianity with a preacher whose Reformed camp has been less than kind to Warren.
The interview was recorded earlier this month and released on Friday.
Piper, well aware of Warren's critics, distanced himself from them.
"Frankly, I'm appalled at the kinds of slander that have been brought against this book by people whose methods of critique, if they were consistently applied to the Bible, would undo it as the Word of God," Piper said, referring to the 2002 book The Purpose Driven Life.
"I'm one of these Reformed types and my types tend to get on your case pretty often."
With some 30 million copies sold and available in over 50 languages, The Purpose Driven Life is the bestselling nonfiction book in history.
Instant fame, however, came with a slew of naysayers. Critics have mainly hit at Warren's doctrine as presented in his book.
But during the interview with Piper, Warren revealed that the book was not intended for nonbelievers. It was written as a workbook to accompany Saddleback Church's 40 days of purpose – an annual spiritual growth campaign.
It was only at the end of writing it that he realized a nonbeliever may come across the book and read it. So he added a few things about salvation.
"If I had known how many unbelievers were going to read it, I would have explained salvation far much more in detail," he told Piper.
Piper, surprisingly, did not find any major problems doctrinally in the book.
"When I read the book, I thought 'what's the issue here?'" he said. And with 20 pages of notes, Piper said he read the book very carefully.
Some of what Warren had to say in the bestseller even made Piper's "spine tingle with gladness."
The aim of the interview was to clarify what Warren believes about such doctrines as the glory of God, the sovereignty of God, unconditional election, eternity and the Gospel, among other things.
On the Gospel and the cross, the 57-year-old pastor said he believes in the imputed righteousness of God.
"Jesus took our payment," the renowned author said. "The reason Jesus came to earth is because the law could not do what we needed it to do and only Jesus could do it."
He also affirmed the five solas – by Scripture alone, by faith alone, by grace alone, through Christ alone, glory to God alone – and said repentance is the fundamental message of Christianity.
"If you listen to my preaching seminar I have an entire session on 'you're not preaching the Gospel unless you're preaching repentance.'"
Warren had been criticized for leaving out repentance in the prayer he wrote for nonbelievers to pray in his Purpose Driven book.
But with repentance meaning a conformity or shift of the mind, though Warren might not have explicitly articulated it in his book, the idea of it was present throughout the entire book, Piper acknowledged.
Warren – who started preaching at 16 – noted that some people view repentance as a change in behavior. But such a change is not the root; it is rather the fruit of repentance, the megachurch pastor contended.
On the sovereignty of God, Warren said one's mistakes, sins and weaknesses are all part of God's plan.
"I would say God custom designed my weaknesses," the Saddleback pastor said. "I'm not making God responsible for my sin. I believe that my own weaknesses are Father-filtered and just as much as God touched Jacob's hip and he walked with a limp the rest of his life, that I have certain emotional weaknesses that are there to keep me dependent upon God.
"How does God make us like His son? I have found both from Scripture and from experience that God allows us in the exact opposite situation in order to teach us character. [For example], God teaches me love by putting me around unlovely people."
Sin is folded into the plan of God also, he affirmed.
He used abortion to illustrate his point. He often says "there are accidental parents but there are no accidental children. You may not have planned your kid but God did."
"And I believe that with all my heart," he said.
God, he said, used the sin of the women listed in the genealogy of Christ for His glory.
Essentially, Warren puts the banner of "what you meant for evil God meant for good" over all sins.
Warren agreed to the interview after he was unable to speak at last year's Desiring God conference as he had previously scheduled.
Piper had drawn some flak last year when he invited Warren to speak alongside some theological heavyweights including R. Albert Mohler and R.C. Sproul. Some supporters of Desiring God complained, arguing that Warren was not on the same page with them theologically or doctrinally. They felt he relied more on pragmatism than biblical doctrine.
While Warren is not known for being a doctrinal preacher, Piper noted that one reason for this "is his intention to be theologically sound and practically helpful without using doctrinal or theological terms in his public ministry."
But inside Saddleback Church, "there is a greater intentionality about building biblical and theological categories into the people's minds and hearts," he affirmed.
Warren made that clear during the interview as well.
"I would take the doctrinal knowledge of any 500 Saddleback members and compare it to any 500 members of any other church and we'd beat them," he said.
Though a Baptist and a Calvinist, Warren refuses to label himself, partly because of the negative connotations.
Rather, he calls himself a "John 3:16 Christian" and an evangelical who believes the doctrines of grace.
He also had a few words for his fellow Calvinists – some of his harshest critics: "I say this in true love, I wish that those who believe in the doctrines of grace would be more gracious."