In The Last Judgment, Michelangelo hid a surprising face. The artist painted St. Bartholomew, recognizable because of the knife in his right hand and the flayed skin in his left. But it wasn’t until 1925 that a doctor spotted Michelangelo’s secret. On the flayed face of the saint, Michelangelo created a self-portrait.

Why would the artist paint himself as an empty skin? Some interpret it as a tragic metaphor for Michelangelo’s soul. Whatever Michelangelo’s motives, he may have painted the flayed skin from life. After all, the artist was a master of dissection.

From one perspective, it almost seems wrong to depict St. Bartholomew with his skin tossed casually over his shoulder. Similarly, showing the martyred St. Lawrence holding a grill to symbolize his end, literally roasted over a fire, could be interpreted as bringing up a sensitive subject.

But for the Christian martyrs, their demise was a badge of honor. By suffering horrific punishments for their faith, the martyrs demonstrated their deep commitment to Christianity. By holding his skin, St. Bartholomew proclaims that he gladly perished for his faith. Wearing the badge of martyrdom proudly also linked martyrs to Jesus, the first Christian martyr.

Many depictions of Bartholomew do not show his gruesome end or portray his skinless body. Instead, the iconography of St. Bartholomew often includes more subtle references to the apostle’s fate.

In Pyotr Basin’s mural of Bartholomew, on display in St. Petersburg, Basin nods to Bartholomew’s fate by including a flaying knife in the saint’s hand. Basin also includes a book, a reference to Bartholomew’s status as one of Christ’s apostles, traveling the world to spread his faith.

Sculptor Marco d’Agrate created the flayed saint who still stands in Milan’s Duomo. In 1562, when d’Agrate made the statue, artists and scholars in Italy devoted themselves to the study of the human form.

Leonardo da Vinci and Michelangelo undertook anatomical dissections to learn about the body. Just years before the Bartholomew statue appeared outside the Duomo in Milan, Andrea Vesalius published his scientific work on anatomy, detailing the muscles and sinews beneath the skin.

In that context, the Bartholomew statue represents 16th-century anatomical knowledge, demonstrating an artist’s ability to realistically capture a skinned man’s appearance.

Being completely flayed alive would certainly end a life. But some images captured a miraculous moment in the story of St. Bartholomew. According to several sources, Bartholomew tossed his skin over his shoulder and began preaching after his executors finished flaying him.

As an 18th-century text claimed: “When [Bartholomew] was stripped of his skin, he still remained alive, and after flaying, preached of the Lord, and only seeing the miracle, many converted.”

Rembrandt painted an almost unrecognizable depiction of Bartholomew in 1661. Recognizable only because of the knife clutched in his hand, Rembrandt’s portrait shows the martyr as an ordinary man. Rather than highlighting Bartholomew’s gruesome end, Rembrandt emphasizes the apostle’s humanity.

The artist may have used one of his own Amsterdam neighbors as a model for the apostle. Rembrandt’s choice to paint Bartholomew as a common man demonstrates the changing religious atmosphere of the 17th century. Instead of painting the suffering martyr, Rembrandt created a melancholy, thoughtful version of the saint.

Images of Bartholomew’s martyrdom rarely show the saint alone. Instead, he’s typically surrounded by two executioners. In many images, the executioners go about their business with little regard for the man’s suffering.

One painting of the martyrdom, in Burgos Cathedral in Spain, shows an executioner casually leaning back on one foot while stripping a man’s skin off his arm. Wearing a headband but not breaking a sweat, the executioner serves as a warning against complacency. The flayer might think he’s just doing his job, but the image captures the danger of complacency.

Bartholomew wasn’t the only Christian martyr, of course. In fact, Christian martyrs make up an entire genre of art. For example, many artists depicted the demise of St. John the Baptist, who had his head removed.

Caravaggio chooses to show the moment in his painting of John’s end. Others present the saint’s head on a silver platter.

Artists also depicted the moment St. John escaped martyrdom. According to a story in the Golden Legend, Emperor Domitian threw St. John into a vat of boiling oil. But the saint remained unharmed by the oil, presumably thanks to God’s favor.