For the past half-century, the Gallup Poll has been phoning strangers, asking personal questions and then telling the world what Americans believe on topics from prayer to haunted houses and the afterlife.

The Gallup Poll's fascination with religion and spirituality has had little to do with the usual rationale for polling -- a client's need to accrue market research data. Instead, the polling giant has been probing the inner life of Americans for a far more personal reason: The boss wants to see souls saved.

"The most profound purpose of polls is to see how people are responding to God," George Gallup Jr. said this month after giving the commencement speech at Gordon Conwell Theological Seminary in South Hamilton, Mass. "When I ask a question on these subjects, what I'm always trying to find out is, 'Are we doing the will of God?' "

On Monday, after 50 years in the family business, Gallup will trade his pollster's cap for retirement. Although he plans to remain influential in the business and active in its outreach projects, he will no longer craft the questions or write the analyses that brought him revered status and provided the basis for 16 books.

Before stepping aside, Gallup grabbed the opportunity to share more than the latest survey data with a graduating class of soon-to-be evangelical pastors. After rattling off a few statistics about U.S. religious practice, the pollster went where he's never gone before in his "Tuesday Briefing" or other official analyses, adding a sermonic spin that suggests where his heart has been all along.

"The world knows a lot about Jesus, but do they know him?" Gallup asked the commencement crowd. "It is for the churches to seize this moment, to take the vague spirituality of the day and turn it into a faith that is solid and transformative."

Gallup, 74, could have become a priest at age 24. But he came to believe that the enterprise founded in 1935 by his father, George Gallup Sr., "could be a ministry." So upon graduation from Princeton University with a degree in religion, he went to work beside his father as an assistant editor. His primary task: writing good questions.

Over the decades, as polling transformed from a wonkish practice of a few eccentrics to a dominant force in U.S. marketing and politics, Gallup wrote questions that would take the nation's pulse on crucial issues: abortion, gun control, the Vietnam War. Through periods of peace and tumult, Gallup said, he never lost interest in also "measuring" what he understands to be the work of the Holy Spirit.

One of Gallup's legacies, colleagues say, is his ability to be rigorously objective in his methodology without disenfranchising himself of his deepest concern. That model has inspired other pollsters.

"It's essential for there to be a human and interested component to survey development," said Nancy Belden, president of the American Association for Public Opinion Research. "When George Gallup Jr. brings an interest in religion and spiritual life to his research, he is going to ask questions that are enlightened by his own thinking. It should not be a source of bias but of improvement to the questions. He's shown how that can happen."

As for the Gallup Poll's future, questions on religion and spirituality are sure to continue, Gallup said, under leadership that shares a keen interest in the topic. And because he still carries his pocket-size notebook, for scribbling down survey questions that might come to him at any hour of the day or night, his ideas might even find their way into a questionnaire now and then.

"The inner life is the new frontier of survey research in coming years," Gallup said. "We know so little about mystical experiences, yet the religious dynamic is perhaps the most powerful of all in American culture. This is a way to unite our country on a deep level and produce a more peaceful world."

George Gallup Jr. was rigorously objective in his polling methodology, colleagues say.