"As someone who is a believer myself, I don’t presume to know someone’s beliefs about whether they follow Christ or not, unless I’ve actually talked with him," the conservative evangelical and son of a Baptist preacher said, according to Time magazine.
So why does Walker continue going down this path? Perhaps it's truly his M.O. when it comes to talking about other people's religion. But two religious experts we spoke with suggested that his position could be politically advantageous in the GOP primaries.
Here's how that works.
First off, it's worth noting here that repeated polling has shown nearly half of Americans are uncertain about Obama's faith, and many — especially conservative Republicans — believe he is a Muslim. A 2012 Pew poll showed just 55 percent of Americans correctly identified Obama as a Christian. Another 17 percent overall and 34 percent of conservative Republicans believed Obama was an adherent of Islam.
That's a significant chunk of the GOP primary electorate — though nowhere near a majority. But it's also clear that most Republicans (like Walker) don't identify Obama as a Christian.
That's the backdrop. So why does that matter to Walker?
Over the past 40 years, Americans have increasingly been letting politics influence their religious beliefs — so much so that they increasingly associate less with their particular church and more with their ideological worldview.
"It used to be an evangelical Protestant would probably not get along very well with a Catholic," said John Green, a political science and religion expert at the University of Akron. "Now, a traditional evangelical and a traditional Catholic have a lot in common."
And that means religion has become a more prominent — and polarized — factor on the presidential campaign trail.
Religious experts theorize that in leaving Obama's faith as an open question, Walker could set himself apart from any number of evangelical Christians also running for president. And by extension, that could reflect on Obama's party — and potentially Hillary Clinton.
"What he's trying to say is that, 'Oh, Mr. Obama and his party . . . they sure are 'other,' " said Stephen Mansfield, the author of the 2011 book "The Faith of Barack Obama."
Religion is nothing new in American politics. From the days of Thomas Jefferson, a candidate's religion has been fair game for attacks. But it hasn't always been a consistent factor on the campaign trail. That changed when Jimmy Carter talked openly about his born-again Christianity in the 1970s and when the religious right rose to prominence via evangelical Baptist pastor Jerry Falwell in the 1980s.
Today, a candidate's religion is a central part of their pitch to voters — so much so that we wouldn't even think of electing a candidate without knowing what they believe about God.
This political-religious landscape means candidates on both sides of the 2016 election have voluntarily put their faith in play. They mention their beliefs frequently in stump speeches, cite the Bible in explaining their positions on controversial topics such as same-sex marriage, and on the right, Republicans seek out religious audiences to make speeches to. (Texas Sen. Ted Cruz even chose conservative evangelical Liberty University — founded by Falwell — to announce his candidacy.)
But while religion frequently takes center stage on the campaign trail, there's little crossover between what Christian Democrats and Christian Republicans emphasize, both Green and Mansfield said.
On the left, both Clinton and Obama adhere to a more liberal, modern brand of Christianity — one that places less emphasis on social issues like gay rights and abortion. (Compare that to past Democratic leaders such as Carter's born-again Christianity, or more recently Bill Clinton, who ran as a religiously moderate Baptist.)
On the right, more Republicans than ever tend to be aligned with theologically traditional brands of Christianity that interpret the Bible more verbatim.
"You've got about as far left and as far right as you can be in Christianity" running for president, Mansfield said.
The candidates' religious polarization reflects that of the average American. A growing share of Americans — 49 percent — say churches should express their views on social and political issues, according to a September Pew Research Center survey.
When viewed through this prism, it makes political sense for Walker — and other candidates — to leave open the idea that Democrats aren't as religiously inclined (or even Christian at all). The stark theological divide between the left and right means that candidates can increasingly invoke religion to mobilize voters for themselves or against the other side.
"The goal is definitely to stick a wedge between the two versions of faith that dominate on the left and the right," Mansfield said.
And Mansfield said you can expect to hear a lot more religious talk from Walker and many other presidential candidates.
"It will come up," he said, "and I think that's probably the way it is for a while in American politics."