On Saturday, his unwilling audience was a group of families snaked around Westgate's seasonal Christmas village at the food court near Hot Topic and the Gap.
Grisham paid no heed to calls to "chill out" from parents - and as his sermon picked up momentum, things were about to get ugly.
"Don't lie to your children and tell them there's such a thing as Santa, when you know in reality that there are no flying reindeer, there is no workshop at the North Pole. . . that you buy all the gifts and put them under the tree."
A girl in a Santa hat stared, wide-eyed. Other children shot confused looks at their mums and dads. And a few parents decided to take action.
A man in a blue T-shirt approaches. He's so close to Grisham that the pastor's cellphone camera captures only his torso.
"Stop," the man says, speaking over Grisham.
"Stop. Stop. Stop. Stop. Stop."
"Quit talking this mess, you understand me," the father says. "I got my kids over there, we don't need you coming over here blabbing whatever the hell you're blabbing."
Other parents approach, diverting Grisham's attention from his message as tensions rise.
This kind of conflict is not foreign to Grisham, the founder of Last Frontier Evangelism. He travels the country with his wife, spreading the gospel to people who are, for the most part, minding their own business.
He told The Washington Post that God spoke to him a decade ago while he and his wife were on vacation in Mexico. He's been street-preaching most weekends ever since.
On Saturday, he whipped out his cellphone camera to record his Santa truther episode, hoping a viral video would help spread the gospel.
He's claiming victory. The video has been seen more than 2 million times. As an added bonus, mall officials say he's not banned from the mall, per se, though they're on the lookout for future inappropriate behaviour.
Grisham said he's planning to do the same thing at other malls before Christmas.
Usually, Grisham and his wife carry large signs that quote scripture and let people know that God isn't fond of whatever they happen to be doing.
"Most of the unsaved people are not going to church," he said. "My wife and I, we go to gay bars, we go to porn shops. If you're going to be a fisher of men, you've got to go where the fish are."
In August, they struck up conversations with people outside a strip club, the Great Alaskan Bush Company, carrying a large sign that said "THE WAGES SIN PAYS IS DEATH."
They spoke at "the Pittsburgh homosexual pride parade," according to a Facebook picture, and at the Republican and Democratic national conventions, venues where people have come to expect a clash of opinions.
The Grishams have also directed their messages at people who have good reason to say they're not actively sinning, like Catholics who traveled to Philadelphia to see the pope.
But Grisham told The Post that his biggest beef this time of year is with Kriss Kringle.
"If Santa Claus was a cartoon caricature like Mickey Mouse and everybody knew that it was fake, and no one thought it was real, I'd be fine with it," he said. "But when you start telling kids that Santa Claus is real, it now becomes idolatry. I'm going after it because it's idolatry."
He's not just "going after" Santa Claus metaphorically.
In 2010, he videotaped a mock execution of a Santa Claus pinata, complete with sound effects and a coup de grace to the Santa pinata.
"What you can't see," he told his listeners, "is that we shot Santa in the face."
