He's been through hundreds of abandoned buildings - churches, schools, silk mills, hotels, factories, an old Hershey chocolate factory that was under demolition, and the plant for the Packard Motor Car Co. in Detroit, one of the largest abandoned sites in North America.
But it all started with an abandoned hospital: Philadelphia State Hospital at Byberry.
When he was in his early 20s, Christopher, whose given name is Michael Murray, worked in the mental health field and was researching how care for the mentally ill had evolved. He found someone to give him a tour of the abandoned facility and was immediately hooked.
That was before Christopher had taken an interest in photography - he didn't bring a camera with him the first time he visited Byberry. He eventually went back with a camera, but he was new to photography at the time, so the quality of that work doesn't quite compare to the pictures he takes today.
One of his biggest regrets, Christopher said, is that he doesn't have any good pictures of Byberry. And he's not going to get them - the building has since been demolished.
After his experience at Byberry, Christopher began to teach himself photography. A few years later, he created a website to highlight his work, calling it "Abandoned America."
But there was a limit to what he could teach himself. So he went back to school, and in 2012 earned a master of fine arts in photography from Rochester Institute of Technology.
In his younger years, Christopher preferred to draw. He didn't have much respect for photography until he tried it out for himself.
"I thought photography was a pseudo-art," he said. "You know, you just point your camera at something and press the button and there you go, it's done."
Now, Christopher said, he knows better.
In spring 2013, he began offering workshops, teaching others how to photograph abandoned buildings and taking them to sites he had scouted in advance.
Christopher gets all levels of skill at his workshops, from people using their camera phones to professionals with equipment more sophisticated than Christopher's own Canon5D Mark II.
Part of Christopher's interest in these buildings is preserving them and what they represent - or at least used to represent - about America's ideology. So he works to give back, donating 40 percent to 50 percent of the proceeds from his workshops to the property owners.
"I would never lie to anybody and tell them that I think what I do is going to win the battle for these places," Christopher said.
In many cases, it won't. But the money can go toward maintaining the building or paying property taxes. The photographer estimates that he has donated $40,000 so far.
"Abandoned America" is about to move from online to print - a book titled Abandoned America: The Age of Consequences is scheduled to be released in November. It will include more than 200 of Christopher's images and will feature many Philadelphia sites.
For Christopher, "Abandoned America" has more than just its literal meaning. When things like schools and mental institutions shut their doors and join the ranks of America's abandoned buildings, so, too, do the ideologies that gave rise to the institutions.
"It's not just about abandoned buildings in America," Christopher said. "It's about the things that made America great or that were fundamental - to at least what my opinion of the concept of America was - that have been abandoned."