Last year the National Geographic Society released “The Gospel of Judas” from a fourth century C.E. manuscript in Coptic, the language of Egypt written in Greek letters. The expense of acquisition had been high, and the fanfare — including a television documentary as well as press conferences and publications — was considerable. Exaggerated claims came with the hype.

To this day, some readers wonder whether “The Gospel of Judas” really was written by Judas Iscariot, the disciple who handed Jesus over to the authorities in Jerusalem. In “Reading Judas” (Viking, 224 pages, $24.95), professors Karen King and Elaine Pagels point out that Judas was dead at least a century before this new Gospel was produced. “It is impossible that he wrote it,” they write. By making that clear, they avoid the false start that has afflicted popular discussion of the document.

Their book deals with “Judas” itself, not the skulduggery to drive the manuscript’s price up, which has been ably described by James Robinson in “The Secrets of Judas” (2006). Ms. King and Ms. Pagels do refer in passing to the damage done to the document in the period after its discovery. We will never understand aspects of “Judas” that would have been apparent 30 years ago: The opportunists who loot artifacts and manuscripts often loot knowledge at the same time. Yet even in a fragmentary state, “Judas” sheds fresh light on ancient Christianity and its sources.

As a religion, Christianity only came into its own during the second century, when believers forged broad agreements concerning which books belonged in the Christian Bible, what specific beliefs were consistent with faith in Christ, how sacraments were to be practiced, and where authority lay in the whole body of the Church. In the midst of all those concerns and others, martyrdom remained a preoccupation.

Christian martyrdom under the Roman Empire was a complex phenomenon. To the authorities, believers were atheists who denied the gods of Rome. Magistrates tried to root out atheism by a variety of means: forcing believers to eat meat that had been sacrificed to a god or engage in the ritual slaughter of sacred animals, and compelling Christians to swear fealty to the emperor as divine son and sometimes to curse Christ. These gambits varied with imperial policy, local circumstances, and the temperament of magistrates. Recourse to torture introduced a wild card. Sometimes the pain promoted acquiescence, while other times it provoked ever more willing deaths for the martyrs’ cause. Complicating all these variables, Jews could claim, as practitioners of a religio licita, exemption from requirement to worship the gods of Rome, and many Christians were either Jews or claimed Israelite identity.

Ms. King and Ms. Pagels refer to studies of martyrdom, but they do not explain its background. Their readers may not appreciate that the issue remained ambient for Christians until the conversion of Emperor Constantine in 312 C.E., and long afterward. Taking these concerns into account, it becomes clear that “Judas” addresses the question of martyrdom. Just as there were tracts that appealed for believers to become willing martyrs, “Judas” was a tract in opposition to martyrdom. The central point in the text is a vivid, surreal vision, in which Judas’s fellow apostles engage in brutal sacrifice, offering their own wives and children in a ritual of human carnage.

During the second century C.E., Irenaeus, bishop of Lyons, wrote a work that he called a refutation of false knowledge. The Greek word for knowledge is gnosis, and it has long been conventional to speak of the people Irenaeus attacked as Gnostics. Gnostics believed that the truth Jesus came to bring was completely spiritual, not material, so that, for instance, they often held that Jesus did not rise physically from the dead, and that the world of matter is a deceptive artifice made by a false god. Ms. King and Ms. Pagels argue that “Judas” represents that perspective.

It was only natural that, believing that this world and its powers were deceptive and material, Gnostics should not be anxious to martyr themselves. Why should they answer to the false authorities of this world, much less suffer for what they said to them? Irenaeus himself drew a distinction between “catholic” believers and Gnostics, on the basis of their willingness to suffer for their faith. In their interpretation of “Judas,” Ms. King and Ms. Pagels are in accord with what Irenaeus said about Gnostics, though they fail to mention this fact.

Instead, the authors go out of their way to insist how ill-informed Irenaeus was, when their own argument demonstrates that though he was biased, Ireneus was also familiar with Gnostic thought. The Bishop was an outspoken critic of the Gnostics and fragments of his work have been discovered in Egypt. The question arises: Was “Judas” in fact a reply to Irenaeus’s insistence that true faith implies a willingness to endure martyrdom? Irenaeus himself refers to a Gospel of Judas, but the content he describes does not correspond well to the present manuscript, which accords better with what the later writer, Epiphanius, calls the Gospel of Judas. This is the kind of detailed issue Ms. King and Ms. Pagels don’t entertain in their brief essay, driven more by advocacy of a modern form of Gnosticism than by evidence.

To make its case against seeking martyrdom, “Judas” relied on a widespread theology, best known in the Epistle to the Hebrews from the Christian Bible. According to the innovative argument of the Epistle, when Jesus died on the cross, he was the perfect sacrifice that accomplished every sacrificial requirement, and therefore obviated all further offering. In “Judas,” that means that Jesus sets aside every form of sacrifice — the ritual of the temple in Jerusalem, the pagan slaughter of animals, as well as Christian martyrdom. That is the reason why Judas helps Jesus to sacrifice himself in this fascinating Gospel.

“Reading Judas” is beautifully presented as well as generally well written, and it sets discussion of Judas along a productive direction. But its failure to bring “Judas” into dialogue with its counterparts in the Christian Bible and patristic writings makes the treatment less engaged with the passionate diversity of the period than it should have been.

Mr. Chilton’s book, “Mary Magdalene: A Biography,” is available from Doubleday.