This is simply too good to be tucked away in storage, DeWitt thought.
The painting was encased in a modern frame; when it was removed DeWitt could see clearly that the portrait panel was nestled into a bigger piece of wood that enlarged the painted surface.
Definitely 19th century, scholars had said. A forgery.
"Lloyd chose to question that," said Mark Tucker, the museum's senior curator of paintings.
The panel is now one of seven similar heads of Christ, drawn from collections worldwide, that are the core of "Rembrandt and the Face of Jesus," which opens at the Art Museum on Wednesday for a run through Oct. 30.
The show - 22 paintings, 17 drawings, and nine prints - is the first in Philadelphia to feature Rembrandt paintings, and the first Rembrandt exhibition of any kind here since 1932, when the Art Museum hosted a show of prints. It began its world tour at the Louvre this year and will continue at the Detroit Institute of Arts.
According to museum officials, it also represents the first time these portraits of Christ have been together since the middle of the 1650s, when Rembrandt declared bankruptcy and many of his most precious possessions were sold to satisfy creditors.
The exhibition strongly suggests that all the small portrait panels, including the one in the Johnson collection, are indeed by Rembrandt van Rijn. Beyond that, these portraits are central to - in fact, are probably studies for - two of the artist's greatest works, also in this exhibition: the Louvre's newly cleaned The Supper at Emmaus (1648) and The Hundred Guilder Print (c. 1649), so called because of its immense value. (One of the versions on view here of the etching, which depicts Christ preaching, is drawn from the Art Museum's own collection.)
DeWitt's questions a decade ago put him in the thick of famously contentious scholarly debates surrounding Rembrandt attributions. They eventually led to a dense technical study of the Philadelphia portrait and several similar portraits of Christ also attributed to Rembrandt's studio. Supper at Emmaus, unquestionably by Rembrandt, served as a kind of benchmark, a scientific control.
Technical analysis of wood panels, groundings, paint, and manner of application showed strong correlations between the portraits and Emmaus. But perhaps most startling, it underscored the bold assertion that Rembrandt's great religious imagery, developed around 1650, marked a radical break from rigid and unquestioned tradition.
"This is Rembrandt's new Jesus," said DeWitt, now curator of European art at the Art Gallery of Ontario. "He invents a new Jesus in the middle of his career."
As a very young painter, Rembrandt portrayed the face of Christ in the traditional northern fashion: thin lips, capacious round brow, sheaves of sandy hair, all echoes of Byzantine icons.
Throughout northern Europe these elements were considered essential to "the true image" of Christ, said DeWitt. "It struck me as profound how strongly even artists in the Netherlands stuck to canonical representations of the face of Jesus."
But Rembrandt, who lived in a luxurious house in Amsterdam's bustling Jewish quarter, determined that the true image of Christ would be found among his neighbors.
"His method of thinking is how to make it truer to life by rejecting tradition, rejecting sources that are [received] sources and instead going to life, finding the truth out for himself by reading the original documents . . . and bypassing the approved layers of tradition," said DeWitt.
Rembrandt, in essence, plucked a Jewish neighbor off the street and had him pose. That seemingly conventional act created a completely new image of Jesus - a Jewish Jesus.
"This was very likely the first time in the history of Christian art that Jesus appeared to be Jewish," said DeWitt.
All the small portraits in the exhibition depict him with "a darker skin, black hair, lower hairline, and a more realistically proportioned forehead" than the traditional iconographic Christ, DeWitt notes in an essay in the exhibition catalog. "This new Jesus has the same broad face, heavy eyelids, and round lips as the younger subjects in Rembrandt's portraits of anonymous Jews."
In an interview, DeWitt said, "Rembrandt makes Jesus human, clearly human, really for the first time. That is breathtaking."
The new Jesus did not sit well with many people at the time. For one thing, Jesus was supposed to be "the most beautiful of men," DeWitt said. For another, Jews were considered "blasphemers."
"It's anti-Semitism, undiluted," DeWitt said.
The realistic portrayal of Christ "was adopted by a few of Rembrandt's pupils, but rejected by others," he said.
"After Rembrandt's own day - tabula rasa. This whole invention goes away. Nobody consciously decides to make their Jesus look ethnographically correct again. It's not until the 19th century that people pick up on that this was something unusual."
In this exhibition, all the heads of Christ are united for the first time, along with Emmaus, and it is possible to see Rembrandt using his model to explore different, very human emotional states.
It is also possible to judge whether the head of Christ at Harvard's Fogg Museum served as a study for Jesus in The Hundred Guilder Print, and whether the portrait at the Detroit Institute of Arts is a plausible prelude to Christ in Supper at Emmaus.
Are these all by Rembrandt? Do they reflect the hand of the master working to gussy up the effort of a pupil? Has a member of Rembrandt's circle, a master painter in his own right (no women here), completely absorbed Rembrandt's technique to the point where there is no way to distinguish between them?
"There are authentic mysteries that will stay that way," said Tucker, the conservator. "That's what it comes down to."
Contact culture writer Stephan Salisbury at 215-854-5594, email@example.com, or @SPSalisbury on Twitter.