As he flew aboard Air Force One to Chicago on his 49th birthday earlier this month, President Obama dialed three Christian pastors to pray with him.

On an airborne conference call, he kidded with the religious leaders about being abandoned by his wife and daughters, who were away on vacation and at camp. As he celebrated his birthday, he was in a reflective mood. He told them he wanted to pray about the year that had passed, what's really important in life and the challenges ahead.

"That was simply something that he wanted to do at his initiative because it was important to him," said Joel Hunter, an evangelical pastor who was on the call and who is part of a small circle of spiritual advisers who frequently talk to Obama by phone.

The prayer session, which was not publicized and which neither the White House nor the ministers sought to bring to light, reflects Obama's decision to keep his public expressions of religious faith to a minimum. Hunter said the president often reaches out to pastors for private spiritual conversation.

Hunter, who is the pastor of Northland Church in Orlando, also advised Obama's predecessor. George W. Bush routinely invoked his own Christian faith in public. Hunter said that Obama's reluctance to do so may help explain the results of a Pew Research Center poll released this week that revealed a sizable increase in the number of people who falsely think that Obama is a Muslim.

Since October 2008, the percentage of Americans who say the president is a Muslim has risen from 12 percent to 18 percent. The percentage of people who think he is a Christian has fallen from 51 percent to 34 percent. The polling data indicated that those who identified themselves as conservative Republicans were most likely to say that he is a Muslim.

"That's the downside of not wanting to expose what is personal to the political arena," Hunter said in an interview. It "leaves all kinds of room for speculation and conspiracy and people who are just not informed."

"You know what happens with a vacuum?" he said."It gets filled."

As the story churned on the Internet and made the rounds of cable news talk shows Thursday, deputy press secretary Bill Burton was left to insist to reporters that the president is indeed a Christian.

"He has spoken about his faith extensively in the past. You can bet that he'll talk about his faith again," Burton said.

Obama's predecessors needed to make no such assurances. Bush made frequent references to his faith. In a Republican debate with John McCain in 2000, Bush was asked what "political philosopher or thinker" he identified with most. He answered: "Christ, because he changed my heart."

Bill Clinton attended church regularly at the Foundry United Methodist Church, a few blocks from the White House.