A decade ago, my wife, Katherine, and I were privileged to meet Billy Graham in his surprisingly ordinary mountaintop home in Montreat, North Carolina.

We found him sitting in his easy chair in his study. As soon as he saw us, he struggled to his feet and asked whether we would like some iced tea.

After he settled back into the chair, his assistant said, “Billy, Grant is writing a book about you.” Graham responded, “Why?”

I wasn’t expecting that question, so I fumbled around for an answer.

“Well, Mr. Graham, you have done some important things.” To that, he offered another unexpected response. “No, the Lord has done some important things through me.”

An hour later, driving back down the mountain and thinking about that conversation, I realized that Graham did not deny that he had done important things. He knew better. Yet he was careful to say that the ability to do important things was not really his own. The Lord had given it to him.

North Carolina’s favorite son turns 99 this year. This landmark birthday seems a good time to pause and think about the reasons for his success. There are many factors, of course. But after studying Graham’s life, I think it’s important to understand the role that personal humility may have played in it.

To be sure, Graham himself was hardly a perfect example of humility. He was a terrible name-dropper, enjoyed basking in the limelight of his friendships with the rich and the powerful, and gave scores of interviews to the press in which he readily spoke beyond his knowledge. He did not seek power for himself, but he relished being close to those who did, sharing in the popular acclaim.

Yet virtually all journalists and historians who encountered him talked about his humility. Even hostile ones who could find little good to say about the man remarked on the striking and undeniable humility he displayed.

Noting the presence of the virtue is one thing. Figuring out exactly what it is and how it shaped Graham’s life and influence is quite another.

To begin with, the concept of humility is elusive. We can’t measure it, as perhaps we can measure generosity. We can’t seek it. The best -- or worst -- book title one could dream up would be “Humility: A Memoir.”

A huge literature on humility comes from classics, philosophy, theology and, of course, the Bible. Type “humility and Bible” into Google and dozens of references immediately pop up.

Some have said that Graham showed humility in his irrepressible self-deprecating humor, and there is a measure of truth in that.

Stories abound. My favorite is one he told about himself in the pulpit countless times. As Graham was riding a hotel elevator one evening, a fellow passenger carefully looked him over, up and down, and then asked whether he was in fact Billy Graham. When Graham nodded yes, the passenger exclaimed, “What an anticlimax!”

Even so, self-deprecating humor may not be a reliable index after all, for to some extent, it was used by Graham as a rhetorical device for warming up a crowd.

Others have pointed to Graham’s readiness to admit his mistakes, large and small. In later life, he repeatedly apologized for falling into the perils of political partisanship in the pulpit, for an occasion when he made odious remarks about Jews and the media in Richard Nixon’s Oval Office, for spending too much time away from his family, and even for watching too much television.

His willingness to apologize undoubtedly won many followers. Yet that too fails as a surefire test of humility. Expressing regret may deflect attention from one’s mistakes and become a substitute for deep self-examination.

So where did Graham’s humility really lie?

I suggest it resided in three qualities. The first was Graham’s habit of self-forgetfulness. Again and again, we see him keeping himself in the background, giving credit to others, especially to his associates, music director Cliff Barrows and soloist George Beverly Shea.

Graham instinctively followed the advice the famed basketball coach Dean Smith gave his players. When you make a basket, Smith said, first thank the guy who tossed you the ball.

The second quality is gratitude. Decade after decade, Graham showed an awareness of the unmerited grace undergirding his life.

When people asked him, as they often did, why so much attention had fallen on him, he invariably responded the same way: “I have no idea.”

The first thing he planned to ask the Lord when he got to heaven, he said, was why the Lord had chosen him, a very ordinary farm boy from North Carolina, to preach the gospel to millions around the world.

And finally, obedience -- or, more precisely, the obedience to a calling that marked his ministry. This meant confidence that the Lord really had called him, that his life was not his own to do with as he pleased.

It also meant confidence in the importance of his vocation. He preached as though people’s lives were at stake, because they were. And confidence in his ability to do the job.

Not to follow his calling would be to dishonor the gifts the Lord had given him.

Graham never disparaged his accomplishments, which would have diminished the work that God had accomplished through him. In this he exemplified the words of Olympic track star Eric Liddell: “I believe God made me for a purpose, but he also made me fast. And when I run, I feel his pleasure.”

At our first meeting with Graham in his home, he left little doubt that he -- like Liddell -- felt pleasure in following God’s summons to preach the gospel.

In these days of bitter division in the culture and in the church, the model of Graham’s personal humility may be as helpful as anything he ever said behind the pulpit.

He certainly was not an angel, if by “angel” we mean someone who made no mistakes in his public ministry. He did make mistakes, and some of them were grievous.

But he was a saint, if by “saint” we mean someone whose life showed the humility of self-forgetfulness, of gratefulness and of faithful obedience to one’s calling. He walked the talk.