Thirty years ago, the art scholar Leo Steinberg published “The Sexuality of Christ in Renaissance Art and in Modern Oblivion,” a book that does much to explain the connection between Pope Francis’s passionate devotion to the poor and afflicted and his seeming openness to gay Catholics. In “The Sexuality of Christ,” Steinberg argues that as a result of the rise of the Franciscan order, around 1260, an emphasis on Christ’s nakedness, and, thus, on his humanity, joined compassion to an acceptance of the role of sexuality in human life.

A credo of the Franciscan order was nudus nudum Christum sequi (“follow naked the naked Christ”). It was a radical call to cast aside worldly wealth and belongings and acknowledge the fragile, fallen nature of all men and women. But in casting aside Christ’s garments, the Franciscans made Christ’s nude body a focal point. As a result, according to Steinberg, from about the middle of the thirteenth century until the sixteenth century artists lavished particular care on Christ’s penis, the part of Christ’s body that made him most mortal, and which proved his union with humankind. “One must recognize,” wrote Steinberg, “an ostentatio genitalium comparable to the canonic ostentatio vulnerum, the showing forth of the wounds.”

“The Sexuality of Christ” has changed the way we look at certain works of art. The “modern oblivion” of Steinberg’s subtitle was just that: centuries during which the central fact of Christ’s phallus in hundreds of Renaissance paintings was overlooked, denied, and, sometimes, bowdlerized. Steinberg adduces several examples of Christ’s genitalia being painted over or touched up to make them look like a mere blur. In one case, probably in the mid- to late nineteenth century, the Alinari brothers, famous for their photographic reproductions of paintings, blackened out the Christ child’s penis in their photograph of a fifteenth-century “Madonna and Child” by Giovanni Bellini. Such censorship, Steinberg believes, was meant as distraction from an uncomfortable theological premise: “A disturbing connection of godhead with sexuality.”

To bring to the surface this suppressed artistic trend, Steinberg reproduced dozens of paintings and drawings in which Christ’s genitalia are indisputably a central thematic concern. There are paintings of the Christ child touching his penis, and of the Virgin handling the infant Christ’s penis. In some pictures, the Christ child exhibits his genitals in a style similar to Venus displaying her sex. “Again and again,” Steinberg writes, “we see the young God-man parading his nakedness, or even flaunting his sex in ways normally reserved for female enticements.”

Many representations of the Three Magi show one of the foreign kings closely inspecting the infant Christ’s genitalia. Depictions of Christ on the cross and of the dead Christ lying in the Virgin’s arms clearly portray Christ with an erection. In some images, which Steinberg calls “psychologically troubling,” the divine Father touches his Son’s penis, “a conciliation,” Steinberg writes, “which stands for the atonement, the being-at-one, of man and God. For this atonement, on which hinges the Christian hope of salvation, Northern Renaissance art found the painfully intimate metaphor of the Father’s hand on the groin of the Son, breaching a universal taboo as the fittest symbol of reconcilement.”

Steinberg argued his thesis with tact, complexity and respect, likely conscious that he was far from the religious tradition he was writing about. He was a Jew, born in Moscow, in 1920—he died in 2011—the son of a man who had been the Soviet Commissar of Justice under Lenin. Disenchanted with the Bolsheviks, Steinberg’s father fled Russia with his family, settling in Berlin and then London. Leo came to the United States in his twenties, where he established himself as an art critic and then as a scholar of Renaissance art, eventually teaching at Hunter College and, later, at the University of Pennsylvania. As an academic, he wrote about art with the speculative and allusive sweep of the reigning art critics of the time, Harold Rosenberg and Clement Greenberg. (The three of them, quipped Tom Wolfe, comprised a potent kingdom of taste called “Cultureburg.”)

Yet Steinberg also had the scholarly heft of another contemporary, Meyer Schapiro, who held court at Columbia. Like Schapiro, Steinberg imbued his reflections on older art with a modernist sensibility. Steinberg was one of those mid-twentieth-century Jewish intellectuals who were drawn to Catholicism. (Schapiro, for example, became an expert in Romanesque art, and Bernard Malamud filled his work with images of Catholic mercy and redemption.) Working as a translator of Yiddish when he first arrived in New York, Steinberg rendered into English the final volume of the Yiddish writer Sholem Asch’s Christian trilogy. In the novel, called “Mary,” Asch explores the relationship between the Virgin and her holy child. For a certain type of Jewish thinker, Catholicism’s beautiful sublimations and dark repressions offered infinite possibilities for dissection and analysis.

“The Sexuality of Christ” takes up, to put it mildly, an ultra-sensitive subject. For that reason, Steinberg stresses that Renaissance artists’ emphasis on Christ’s penis is an esthetic choice guided by deep religious belief, though he occasionally hints that Renaissance artists could at the same time have been having sly fun with the subject. And it is hard to believe that in, say, quattrocento Florence, an epoch so liberated in its sexual mores—Fra Filippo Lippi, for example, lived openly with a defrocked nun, whom he used as a model for his Madonnas—artists could resist being simultaneously worldly and pious.

For Steinberg, however, theological motives were preëminent. He held that artists used the evidence of Christ’s genitals to prove that Christ submitted to becoming human before returning to the godhead. The revelation of his penis demonstrates, as Steinberg puts it, Christ’s “humanation,” that moment of incarnation which proved Christ’s love for humankind. And the many representations of the Christ child’s circumcision are important as foretellings of his crucifixion—the blood of Christ’s penis is fulfilled in the blood from Christ’s wounds.

Entering with obvious relish the realm of Christian sexual hermeneutics, Steinberg relies on St. Augustine, who emphasized his surrender to and then escape from the “fleshpots of Carthage,” to argue that Christ’s erection was a singular way to demonstrate Christ’s chastity. Without the capacity to yield to lust, Christ’s triumph over carnal desire would have no human meaning. Unlike men after the fall of Adam, who fell victim to lust, Christ willed his erection; it was not an involuntary physiological event. By both willing and resisting it, he declared his victory over the stain of sin bequeathed to humanity by Adam and Eve, and over the death that their carnal weakness brought into the world. That, after all, is the significance of the resurrection.

To drive this point home, Steinberg had to prove that during the late Middle Ages and the Renaissance the word “resurrection” could be used as a double entendre, connoting both the divine event and the humble mortal fact of an erection. Steinberg quotes from one of Boccaccio’s fourteenth-century tales in the Decameron, in which a pious young girl inflames the desire of a monk named Rustico, causing in the latter a “resurrection of the flesh.” Steinberg notes that it was not until modern times that the original phrase was accurately translated from Italian, a censoring that he sees as analogous to the later bowdlerization of Christ’s penis in Renaissance paintings. (You can find a faithful rendition of Boccaccio’s phrase in the superb translation of the Decameron from Norton; it occurs in the tenth story of the third day.) Here, as throughout his argument, Steinberg hints at the painters’ playfulness about the sacred subject, but he leaves it at that.