CNT = Bruce Metzger, The Canon Of The New Testament (New York: Oxford University Press, 1997)
FGO = Martin Hengel, The Four Gospels And The One Gospel Of Jesus Christ (Harrisburg, Pennsylvania: Trinity Press International, 2000)
JAE = Richard Bauckham, Jesus And The Eyewitnesses (Grand Rapids, Michigan: Eerdmans, 2006)
JCO = Steve Mason, Josephus, Judea, And Christian Origins (Peabody, Massachusetts: Hendrickson Publishers, 2009)
TCD = Lee McDonald and James Sanders, edd., The Canon Debate (Peabody, Massachusetts: Hendrickson Publishers, 2002)
TJL = Paul Eddy and Gregory Boyd, The Jesus Legend (Grand Rapids, Michigan: Baker Academic, 2007)
Each generation builds on the knowledge of previous generations. In many ways, we're better able to evaluate history than those who lived in the past. But they had some advantages over us as well, and future generations will have advantages over previous generations, including ours.
The early Christians were often wrong, but they were more often right. It should be kept in mind that when people criticize an error in a Christian writer of the second century, for example, that same writer is correct on thousands of other issues he addresses directly or indirectly. Often, a focus on errors distorts our judgment of the overall reliability of a source. In the past, I've written about such distortions in the context of human memory.
People know more about some subjects than others. A second-century Christian in Italy may be highly ignorant of the history of ancient Egypt or events in Greece hundreds of years before he was born, yet be much more knowledgeable about events in his own region of the world a generation before his birth or events within the church of his day. Similarly, a modern American may be highly ignorant of Japanese history, yet know much more about American history. We don't dismiss the testimony of a witness in a court of law, concerning a murder he claims to have seen, because he believes in ghosts or carries a good luck charm. We should ask questions such as what interest a source has in the subject under consideration, what his standards of evidence are, and what evidence he had access to.
Despite our advantages over people of the ancient world in some contexts, we have access to only a portion of the evidence that the early Christians had in making judgments relevant to the New Testament canon. Martin Hengel observes that "of the second-century Christian writings known to us by title, around 85% have been lost. The real loss must be substantially higher." (FGO, 55)
Consider New Testament manuscripts. (Some of the issues related to manuscripts are relevant to the canon, such as gospel titles and whether chapter 21 of John's gospel was part of the original document.) We often argue over how much a manuscript of the late second or early third century reflects the text of the early second century, whether earlier manuscripts are likely to have had the same titles as the later manuscripts we possess, etc. But Christians of the first two centuries had easy access to a large number and variety of such manuscripts, far more than we possess today. Men like Polycarp, Justin Martyr, and Irenaeus would have regularly come into contact with manuscripts that predated what we have today, sometimes including manuscripts contemporary with the apostles or disciples of the apostles. Bruce Metzger notes that some patristic sources refer to the preservation of some of the original copies of the New Testament documents (CNT, n. 4 on 4-5). Metzger cites the example of Tertullian's claim that the church of Thessalonica still possessed the original copies of the letters Paul sent them.
Though we today only have the writings of several disciples of the apostles, there obviously would have been a far larger number of such people who lived in the first and second centuries. Thus, Irenaeus can comment that "there were many still remaining who had received instructions from the apostles" during the lifetime of Clement of Rome (Against Heresies, 3:3:3).
Contrary to the common assertion that the early Christians weren't concerned with evidence or had little concern for it, their belief system and their system of church government were founded on evidential concepts, such as fulfilled prophecy and eyewitness testimony. The highest church office, that of apostle, consisted only of eyewitnesses, and the churches that had a historical relationship with the apostles were the most prominent in the second century (Rome, Smyrna, Ephesus, etc.). Bruce Metzger refers to the "'authority' that underlay the impulse to form a canon of Scripture - an authority that rested upon the testimony of eye-witnesses....As Prorector of the University of Utrecht, van Unnik delivered a learned address on the status of an eye-witness and an ear-witness in vouching for trustworthiness of what was included in early collections of New Testament books." (CNT, 28-29)
Concerning one of the earliest church fathers and one of our earliest sources on canonical issues, Harry Gamble writes:
"Among other things, it can now be seen that Papias's interest in 'the living and abiding voice' does not signify that he was wholly committed to oral tradition and ill-disposed to written materials, but rather, in accordance with a widespread topos, that he preferred firsthand information. Indeed, he should be understood to invoke oral tradition precisely to legitimize written Jesus tradition." (TCD, 278-279)
See, also, Richard Bauckham's case for the general reliability of Papias (JAE, 12-38, 202-239, 412-437). Justin Martyr refers to the importance of evidence, including hostile corroboration (First Apology, 20, 30, 33-34, 53). Tatian is aware of the value of hostile corroboration (Address To The Greeks, 31) and firsthand knowledge (Address To The Greeks, 35). Tertullian appeals to information in the registers of apostolic churches (The Prescription Against Heretics, 32). Dionysius of Alexandria evaluates the New Testament books on the basis of their internal evidence, making some of the same observations that have been made by modern scholarship (Eusebius, Church History, 7:24-25). Eusebius appeals to internal evidence as well (Church History, 3:25). Etc.
Irenaeus is a good illustration of some of these points. He had access to other churches and men of the previous generation. His predecessor in the bishopric of Lyons, a man named Pothinus, died beyond age ninety in the late 170s (Eusebius, Church History, 5:1:29). He was a contemporary of the apostles at a young age and a contemporary of the apostles' disciples as a grown man. Irenaeus would have had access to Pothinus, Polycarp, and probably many other men who could have given him significant information about canonical issues. It's highly unlikely that he would have misunderstood what all of those people said or would have lied about it, for example. It's even more unlikely that all of the sources who corroborate Irenaeus were mistaken as well.
Concerning his relationship with Polycarp, a disciple of multiple apostles, Irenaeus wrote the following to a contemporary who had also met Polycarp:
"For, while I was yet a boy, I saw you in Lower Asia with Polycarp, distinguishing yourself in the royal court, and endeavouring to gain his approbation. For I have a more vivid recollection of what occurred at that time than of recent events (inasmuch as the experiences of childhood, keeping pace with the growth of the soul, become incorporated with it); so that I can even describe the place where the blessed Polycarp used to sit and discourse— his going out, too, and his coming in— his general mode of life and personal appearance, together with the discourses which he delivered to the people; also how he would speak of his familiar intercourse with John, and with the rest of those who had seen the Lord; and how he would call their words to remembrance. Whatsoever things he had heard from them respecting the Lord, both with regard to His miracles and His teaching, Polycarp having thus received information from the eye-witnesses of the Word of life, would recount them all in harmony with the Scriptures. These things, through God's mercy which was upon me, I then listened to attentively, and treasured them up not on paper, but in my heart; and I am continually, by God's grace, revolving these things accurately in my mind. And I can bear witness before God, that if that blessed and apostolical presbyter had heard any such thing [as what heretics are saying], he would have cried out, and stopped his ears, exclaiming as he was wont to do: 'O good God, for what times have You reserved me, that I should endure these things?' And he would have fled from the very spot where, sitting or standing, he had heard such words. This fact, too, can be made clear, from his Epistles which he despatched, whether to the neighbouring Churches to confirm them, or to certain of the brethren, admonishing and exhorting them." (Fragments, 2)
Notice the appeal to an eyewitness of the apostles. The appeal to corroboration from other sources. The appeal to written records that we no longer have today. (We only have one of Polycarp's letters.)
Elsewhere, Irenaeus appeals to the historical witness of multiple apostolic churches and individuals associated with those churches who had been eyewitnesses of the apostles (Against Heresies, 3:3:1-4). While discussing a dispute about the text of Revelation, he appeals to "ancient copies" of the book and the testimony of those who "saw John face to face" (Against Heresies, 5:30:1). Note the appeal to multiple earlier copies of the document that were "ancient" from the perspective of a late second-century writer and the appeal to multiple eyewitnesses of John.
Going back a generation further, consider the influence of Polycarp on New Testament authorship attributions, for example. When Polycarp visited Rome in the middle of the second century to discuss some issues of controversy with the Roman bishop Anicetus, for instance, he surely would have interacted with the beliefs of the Roman Christians on issues of New Testament authorship. We have some idea of how those documents were used at that time in Roman church services, from sources like Justin Martyr. Would documents like the gospels and the letters of Paul have been used without any reference to their authorship? When controversial issues arose, like the ones Polycarp discussed with Anicetus, New Testament documents would have been cited in the process. The concept that somebody like Polycarp could live for several decades as a Christian and travel and involve himself in teaching and controversies, as he did, yet have little effect on the authorship attributions of his day, is untenable.
Sometimes an objection will be raised to the effect that the claims of the early Christians are unreliable, since they don't cite sources or give us a detailed account of how they reached their conclusions. But when an ancient Christian does go into such detail, as Irenaeus does above regarding Polycarp as one of his sources, critics often dismiss what he said anyway. And it was unusual for ancient writers in general, not just Christians, to go into depth about their sources or the process by which they arrived at their conclusions. The same is true today. Imagine the untenable length of Luke's writings, for example, if he cited sources for every historical claim he made, accompanied by a detailed explanation of how he reached each conclusion. Richard Bauckham notes that "Historians in antiquity did not name their eyewitness sources as a matter of course, but in specific cases they did" (JAE, n. 35 on 304). Paul Eddy and Gregory Boyd, while commenting on a passage in the Roman historian Tacitus in which Tacitus doesn't cite a source, observe that making historical claims without citing a source is "a practice that was typical for ancient historians" (TJL, 184). The Josephan scholar Steve Mason notes that the Jewish historian Josephus usually doesn't identify his sources (JCO, 11). He also notes that it was common for ancient writers to leave details out of their writings when they could assume knowledge of those details on the part of their audience (JCO, 50). He cites the comment of an ancient source, Demetrius, who wrote that "It is a slur on your hearer to tell him everything as though he were a simpleton." (JCO, 50)
Much more could be said. I've addressed topics like these many times in the past, such as here and here.
I close with some comments from Bruce Metzger and Martin Hengel, regarding the canonical judgments of the early Christians:
"In general, these [non-canonical] gospels show far less knowledge of Palestinian topography and customs than do the canonical Gospels...one can appreciate the difference between the character of the canonical Gospels and the near banality of most of the gospels dating from the second and third centuries....these four [gospels] came to be recognized as authentic - authentic both in the sense that the story they told was, in its essentials, adjudged sound by a remarkably unanimous consent, and also in the sense that their interpretation of its meaning was equally widely recognized as true to the apostles' faith and teaching. Even the Gospel of Peter and the Gospel of Thomas, both of which may preserve scraps of independent tradition, are obviously inferior theologically and historically to the four accounts that eventually came to be regarded as the only canonical Gospels....the apocryphal Acts cannot be put on a level with the Lucan work...The knowledge that our New Testament contains the best sources for the history of Jesus is the most valuable knowledge that can be obtained from study of the early history of the canon." (CNT, 167, 173-174, 180, 287)
"[Irenaeus gives us] historical arguments which must be taken seriously....At the same time, according to all our historical knowledge and an impartial, sober comparison between the apocryphal Jesus traditions and the four Gospels, indeed the New Testament generally, the church of the second century could hardly have made a better choice....To emphasize the point once again: in its selection and ordering the church of the second century showed historical and theological understanding. I would like to repeat emphatically here the remark made above (33): the church really could not have made a better choice." (FGO, 33, 115, 140)