For more than four decades, New Testament scholars have been discussing the “Secret Gospel of Mark.” In 1960 Morton Smith, a professor at Columbia University, announced the existence of this document at a meeting of the Society of Biblical Literature, a year and a half after he said he found it in a monastic library near Jerusalem. Press coverage proved wide and instantaneous, because “Secret Mark” climaxes with an evocative image: A young man who wore only “a linen cloth over his naked body” spends the night with Jesus, who “taught him the mystery of the kingdom of God.” That proved too good a lure to pass up: What reader of the Gospels could fail to wonder whether Jesus engaged in the sexually charged initiation that “Secret Mark” describes? Smith himself, a homosexual at a time when homophobia ran high, had little doubt.

That controversy got in the way of resolving complications that should have been sorted out from the beginning, because this text is not an ancient manuscript at all. “Secret Mark” is supposed to be an 18th-century copy of a letter written by the second-century theologian Clement of Alexandria. The letter details and criticizes the teaching of the Gnostic philosopher Carpocrates and quotes esoteric material from this “secret” version of Mark’s Gospel that adds extra stories, such as the initiation of the naked young man, to the original.

Many groups during the second century, like the one that gathered around Carpocrates (an actual teacher), claimed they understood the truth about God and Jesus by means of a special gnosis, the Greek word for “knowledge.” They sought the gnosis that would take them out of the sufferings of this world and bring them to heaven, and for that reason they came to be known as Gnostic. Gnosis for them wasn’t just a collection of data or reasoned argument, but insight into the celestial realm that transformed the Gnostic seeker. The true Gnostic transcended the shackles of the fallen, physical world and became inured to suffering and pleasure in his or her total dedication to the spiritual life. Each person who found Gnosis lived thereafter in the assurance of divine favor, saved from the corruption of the flesh, incarnated within the realm of spirit.

Gnostic literature came into prominence as a result of a genuine discovery, of the Nag Hammadi library in Egypt in 1945, at a time when historical acumen was unfortunately on the wane in the United States. As a result, Gnostic sources have been routinely confused with history, and some documents that are obviously from the second and third centuries (“The Gospel of Thomas,” “The Gospel of Philip,” “The Gospel of Mary,” and most recently “The Gospel of Judas,” for example) have been touted as reporting the truth of the story that the New Testament supposedly distorts. “Secret Mark” fed this naïve enthusiasm, and profited from it.

Publicity and naïveté have encouraged the rise of a form of neo-Gnosticism, a fashion greatly encouraged by recent discoveries and alleged discoveries. In embracing these ancient sources, the neo-Gnostics are unlike their ancient counterparts. They want to embrace the earth, not subjugate it; they don’t wish to be elitist. Above all, they want to insist on the gender-equality of women with men. You need to cherry-pick Gnostic sources, and ignore a great deal of what they say, to make that picture work as an account of the Nag Hammadi library. Neo-Gnostics do just that, and falsify history. Many ancient Gnostics were openly anti-Semitic, taught that the physical world was the hopelessly corrupt product of a false god, and insisted that only the predestined elect could know the divine truth. These are persistent tendencies, rather than a set of precise ideas that all Gnostics repeated, but they are facts that can’t be denied.

Since Smith’s alleged discovery and its endorsement by neo-Gnostics, “Secret Mark” has influenced a generation of scholars and found a big popular audience. Within the “Jesus Seminar,” which claims to be searching for the historical Jesus, John Dominic Crossan championed “Secret Mark” as authentic, making it a cornerstone in his portrait of Jesus as a deliberately anti-conventional philosopher in the style of the Greek Cynics. Helmut Koester at Harvard has even claimed that “Secret Mark” is an earlier version of the Gospel according to Mark in the New Testament. Elaine Pagels wrote a glowing foreword for a reprint of Smith’s book for Dawn Horse Press, the publishing wing of a movement guided by the self-designated Avatar Adi Da Samraj, who claimed to continue Jesus’s sexually liberating practices.

To untangle the claims about “Secret Mark,” scholars need to ask themselves whether Clement of Alexandria wrote the letter, then whether Clement got Carpocrates’s teaching straight, and then — and only then — whether “Secret Mark” tells us anything about Jesus and the formation of the Gospels. But reactions to the image of a homoerotic Jesus shortcircuited common sense as well as sound professional judgment four decades ago, and continue to do so today.

Partisan scholars opposed to “Secret Mark,” for the most part conservative evangelicals, have dismissed the document and those who support its picture as “radical fringe.” Because they often do so before dealing with any evidence, they inevitably seem uncritically defensive of orthodox Christianity. Proponents of “Secret Mark,” on the other hand, have contended that Carpocrates’s teaching represents an early version of Mark, prior to what is in the New Testament, and that Jesus and his followers engaged in esoteric — and sexual — practices.

A recent book by Stephen C. Carlson shows us how the basics of scholarship were eclipsed by sensationalism on the left, compounded by willful dismissal on the right, and why “Secret Mark” needs to be seen as a fraud. In “The Gospel Hoax: Morton Smith’s Invention of Secret Mark” (Baylor University Press, 151 pages, $24.95), Mr. Carlson, a lawyer, argues his case as if in a civil proceeding, meeting the test of proof by preponderance of evidence, rather than beyond a reasonable doubt. He has mastered his brief impressively, and although in my view he does not quite prove that Smith was a forger, he does demonstrate — within the limits of certainty that incomplete evidence involves — that “Secret Mark” is someone’s forgery, and that Smith, who died in 1991, was the likely culprit.

The physical evidence of “Secret Mark” has always been problematic. Smith presented fuzzy photographs he took of the original document in 1958 when he eventually published full studies of it in 1973.These and later images in color constitute the only material evidence for the existence of the document, which was moved from the Mar Saba monastery to a library in Jerusalem in 1977. The Greek Orthodox Patriarchate, the legal custodian, has not released the text for further study; its use in the West to portray Jesus as homoerotic probably explains why. But even on the basis of the photographs, Mr. Carlson shows there are signs that the copyist hesitated, lifted the pen, and retouched in the course of forming letters — all telltale clues of forgery. Mr. Carlson further suggests that the fuzziness in the photographs represents what would happen if a person were to write on old, porous paper.

Handwriting from the same scribe, in Mr. Carlson’s view, also shows up in another manuscript in the monastery. A 20th-century Greek copyist named M. Madiotes added to the collection, and Mr. Carlson argues that Morton Smith invented this name, and was himself the copyist. At this point, argumentative skill gets ahead of Mr. Carlson’s expertise. He claims that “Madiotes” is a made-up name that means “swindler” or “baldy” (and Smith was indubitably bald). In fact, although uncommon, this family name is still in use; and it is as parlous to press proper nouns for meaning in Greek as it is in any language. Mr. Carlson has opened a possibility, but only an inspection of the original documents could substantiate identifying Smith with Madiotes, and as the writer of “Secret Mark.” Nonetheless, Smith’s contention that the text was copied during the 18th century has lost any basis. And as Mr. Carlson shows, anyone with a familiarity with Greek of the second century and the writings of Clement of Alexandria might have produced the wording of the letter.

But why would anyone, in any century, produce “Secret Mark”? Mr. Carlson persistently calls the document a “hoax” rather than a fraud or a forgery, on the grounds that a hoaxer does not directly profit from his deception. That defense is only partially convincing. Smith’s generation saw several famous cases of hoaxers who deliberately showed up weaknesses in their disciplines and their own cleverness by taking credit for their tricks. One case at the Episcopal Theological School in Cambridge involved faking a biblical manuscript and getting it past several experts, a joke that is memorialized in a stained glass window in the chapel. But those hoaxers — who included Reamer Kline, later the president of Bard College — admitted their prank; it was part of the fun, although some of them admitted to embarrassment in retrospect. Smith did not reveal his trick (if it truly was his) during his lifetime, nor did he arrange for any disclosure after his death. And he profited from publication and publicity; “Secret Mark” set the stage for the reception of Smith’s own idiosyncratic ideas about Jesus.

This brings us back to that night Jesus allegedly kept company with his all but naked disciple. The scene promotes the view that Jesus engaged in magical procedures of initiation that were sexual in nature. Smith had an interest in ancient magic as well as in paleography, and before “Secret Mark” came on the scene he had argued — by mere assertion — that Jewish mysticism during the first century involved homoerotic practices. In just a few phrases, Carpocrates in “Secret Mark” confirmed Smith’s thesis and the laid groundwork for claiming that Jesus championed homosexuality.

In fact, the brevity of the alleged confirmation the “Secret Mark” provides to the “gay Jesus” of recent fashion betrays where it came from. That the young man “remained with [Jesus] the night,” the focus of discussion for the past half-century, reflects a simple phrase in ancient Greek, in which the words mean just what they say. The euphemism that gives them sexual significance is modern, as Mr. Carlson explains. To his mind, that is proof that Mr. Smith made the text up. But whether Smith did forge the “Secret Mark” (which I regard as most plausible, if not proven), he would have been well aware that ancient writers could conceive of nighttime sojourns without sexual contact taking place. Part of the cleverness of the fraud follows from a meaning that 20th-century readers naturally impute to the words of a putatively second-century document.

Smith surely realized that the Greek text would not indicate a sexual liaison except in a modern understanding of its words. This explains why, in a later book called “Jesus the Magician,” he does not refer to the “Secret Gospel” as offering proof for his position. Mr. Carlson sees that as Smith’s implicit recognition of the forgery. That is possible, although I do not think Mr. Carlson has proved that Smith personally forged the “Secret Gospel.” But he has shown that Smith had every reason to know that both the text and the sexual meaning attributed to it are spurious. That he used “Secret Mark” in the way he did ruins his reputation as a scholar.

Smith suffered insults from his colleagues during his career as a result of his sexual orientation, and he believed that prejudice explained his denial of tenure at Brown University. Mr. Carlson suggests that he decided to take his revenge by inventing and publishing his transcription of the “Secret Mark.” Smith also wrote a dense, often rambling commentary that Harvard University Press published; if he hoped to advance his career thereby, he had no luck. But professional setback and frustration no more justify fraud than calling his trickery a hoax, rather than the fraud it is, makes it palatable.

The pattern of public discussion of the “Secret Gospel” is all too familiar: a discovery is claimed that is trumpeted in the press, only to be discredited by scholarly discussion that is then ignored. The result is that palpably misleading ideas — in this case, a Greek Jesus complete with platonic sexual proclivities and a Gospel tradition shorn of its Semitic background — continue to be repeated long after critical credibility has been exhausted.

No literature has suffered more from this problem than that of the second century of Christianity. In the case of “the Secret Gospel,” a modern researcher (Morton Smith himself, or someone whose forgery duped Smith) has made up a Gnostic document up in the attempt to manipulate scholarly discussion and public perception. The fact that this attempt succeeded for so long stands as an indictment of American scholarship, which prides itself on skepticism in regard to the canonical Gospels, but then turns credulous, and even neo-Gnostic, when non-canonical texts are concerned.

Mr. Chilton is the Bernard Iddings Bell Professor of Philosophy and Religion at Bard College.