That's the simple, if too glib, answer. And the authors, who have taken a good deal of time and care to carefully examine the question, deserve a better response than that. Still, as a former Protestant Evangelical who entered the Catholic Church on the Easter Vigil of 2005, that's the conclusion I came to.
I came to the Catholic Church because I arrived at the point where I could affirm her self-understanding. This came about through a thirty-year process where I looked at the questions dividing Evangelicalism and Catholicism from the point of view of history, theology, and practice. Since Mark Noll is a historian, he seems especially attuned to the strength of the Catholic position, and the weakness of the Evangelical position, vis-a-vis history. Anyone who looks closely at the history of the Church in the first few centuries following Christ's resurrection will see clearly that it very early on takes on a Catholic appearance. From Clement of Rome through Ignatius of Antioch through Polycarp through Justin Martyr through Iranaeus--that is, from about the end of the first century through the end of the second century--the Church increasingly comes to resemble its present shape, in its structure, ecclesiology, liturgy, theology, and sacramental understanding. This is so clearly established that no one, except Protestant liberals like Elaine Pagels and Bart Ehrman, questions it anymore. The difficulty for Protestant Evangelicals is that they accept the theological developments but not the structural, ecclesiological, liturgical, and sacramental developments. The question arises, why accept the one and reject the others? How is it that the same Church that is developing proper Trinitarian and Christological understandings, as well as determining the canon of Scripture, can be right in the one area and wrong in the other areas? What principal is at work here? The problem with Protestantism is that there are no historical antecedents for it. That is, it can't be shown to have existed before the 16th century. The question then arises, If Protestantism is true, why did it take the Church 1500 years to find it out? For some strange reason, Protestants seem to have seldom asked themselves this question. Indeed, as recently as 1953 we have the noted Protestant theologian and historian Oscar Cullman writing: "We, on the Protestant side, are beginning to understand the immense wealth that is contained in the writing of the Church Fathers and are beginning to rid ourselves of that strange conception of the Church's history that claims that, with the exception of a few sects, there was a total eclipse of the Gospel between the second and sixteenth centuries." The remarkable thing to me about this quote is that it acknowledges that it took Protestantism 400 years to discover it had no heritage, except the writings of the Catholic Church, which it initially rejected.
On the theological side, N. T. Wright (along with Ed Sanders and Ben Meyer) has definitively shown in What St. Paul Really Said and in The Climax of the Covenant that Paul can't be made to have said what the Reformers said he said, namely, that Justification should primarily be understood as the divine imputation of Christ's righteousness to the sinner, taking place under some heavenly juridical circumstances. Indeed, as Noll acknowledges, Protestants are coming to understand the difficulties in sustaining a Reformed understanding of Justification, just as they are coming to realize that Catholics have always affirmed that it is God who Justifies sinners. The dispute really hasn't been about Justification as much as it has been about what Faith means. For Protestants, Faith has traditionally meant assent to the Gospel message; for Catholics, Faith has meant not only assent to the Gospel message but also entrance into the Body of Christ and faithfulness to it and its Head, Jesus Christ. From a Catholic perspective, it appears that the authors have a fairly adequate understanding both of the Catholic position and what still prevents full acceptance of it by Evangelicals, but they seem insufficiently aware of the erosion to the classic Protestant position that has occurred as a consequence of the work of Wright and as a result of a better formulation of the Catholic position by its theologians.
The authors do seem to have grasped the idea that what most deeply and significantly divides Protestant Evangelicals and Catholics is a different concept of the Church. But even here, I'm not sure they've really put their finger on it. For a Catholic, the unvarying record of Scripture and early Church writings establishes beyond a doubt that Jesus Christ intended not only to establish the Church as a visible, institutional continuation of his ministry, but passed on to it some of the divine prerogatives of his ministry (always, it must be remembered, acting in His name, He Himself being the Real Minister), such as the keys, the forgiveness of sins, and the consecration of the Eucharistic gifts. Protestants do not believe the Church has been given such prerogatives, nor do they believe it was established by Christ, despite the fact that Catholic understandings about the Church, its structure, sacraments, and ministry arose very early and without dispute within the Church itself. For a Protestant the Church is a consequence of the believer's Justification by God by divine decree. It is the fellowship of all those who share this experience of Justification. The problem with this position is that it reduces Christianity to a theory of how Atonement works. The Catholic position, on the other hand, is rooted in an understanding that Christianity is the coming to fruition of God's eternal plan worked out in history in the person of Jesus Christ and continued in his divine/historical Body, the Church.