It's often suggested that we have no reason to think that what we today call orthodox or traditional Christianity was superior to its earliest rivals who also claimed to be Christian. Why prefer traditional Christianity to Gnosticism? Or Marcionism? Or other early alternatives? How could we tell that one group has a better claim to truth than the others?
There are a lot of ways to compare these groups. We can look at their size, the character of their leaders, what evidence the groups offer for their claims, the internal consistency of their claims, etc. What I want to do in this post is discuss several examples.
There's a lot of evidence that the people we today call orthodox were the majority of professing Christians in the earliest centuries. Those people are more prominent in the historical record, including in descriptions of Christianity in non-Christian sources, and all agree that they were the majority in later centuries. Their majority status in the earlier centuries would make more sense of their later prominence.
C.E. Hill has argued that the manuscript evidence suggests that the canonical gospels were much more popular, and represent a much larger number of ancient professing Christians, than heretical and apocryphal gospels (Who Chose The Gospels? [New York: Oxford University Press, 2010], 7-33). See, also, here.
Even Celsus, who often spoke hyperbolically of Christian diversity and sectarianism, recognized that there was a "great church" and "those of the multitude" (in Origen, Against Celsus, 5:59, 5:61), probably the orthodox mainstream that sources like Irenaeus and Tertullian refer to. Celsus may have a similar concept in mind concerning Judaism when he refers to "the multitude of the Jews" (ibid., 5:61).
When addressing heretics in the second century, Irenaeus often refers to them as highly fragmented and suggests that they were relatively small groups. As Eric Osborn noted, "He [Irenaeus] contrasts the universal spread of the rule [core doctrines of orthodoxy] with the local sectarian Gnostic phenomena." (Irenaeus Of Lyons [New York: Cambridge University Press, 2005], 23)
Irenaeus tells us that some heretics rejected some New Testament documents (Against Heresies, 3:11:7), but that most "do certainly recognise the Scriptures; but they pervert the interpretations" (Against Heresies, 3:12:12). In his homilies on Luke's gospel, Origen notes, "There are countless heresies that accept the Gospel According to Luke." (Joseph Lienhard, trans., Origen: Homilies On Luke, Fragments On Luke [Washington, D.C.: The Catholic University Of America Press, 1996], 67) Harry Gamble writes:
"This means that what was at stake between gnostic and non-gnostic Christians was not principally which books were authoritative, but rather how the scriptures were to be rightly interpreted. In point of fact, gnostic Christians employed virtually all the books that were used in the church at large. The difference lay not in the documents, but in different hermeneutical programs." (in Lee McDonald and James Sanders, edd., The Canon Debate [Peabody, Massachusetts: Hendrickson Publishers, 2002], 293)
Irenaeus appeals to evidence of a highly public nature to argue for his view of Christianity, whereas the Gnostics appeal to evidence of a highly private nature. That's a significant difference that suggests orthodox Christians had more objective evidence for their belief system. Bruce Metzger wrote:
"The Gnostics acknowledged this [that Gnosticism wasn't found in the New Testament documents], but asserted that such teachings had not been communicated by the Lord to the general public, but only to his most trusted disciples....The Gnostics also produced other texts in which the apostles report what the Lord had secretly communicated to them....Alongside such 'secret' traditions the Gnostics would, naturally, also know and even utilize the books received by the Church, while interpreting them in their own special manner." (The Canon Of The New Testament [New York: Oxford University Press, 1997], 77-8)
Elsewhere, Metzger discusses how the New Testament documents are more accurate than the heretical and apocryphal literature in what they say about history, topography, and other subjects (ibid., 167, 173-4, 180, 287).
Notice that Gnostics thought they had to concede the authority of the New Testament documents, whereas traditional Christians saw no need to concede the authority of the Gnostic literature. And notice that Gnostics had to appeal to dubious interpretations of the New Testament in an attempt to reconcile the documents with their beliefs. They weren't interpreting the New Testament with the same sort of historical-grammatical method of interpretation that's normally applied when reading literature. How many of today's skeptics would argue that the Gnostics had an equal or more accurate interpretation of the New Testament? Few, I suspect.
When Marcionites acknowledge that they disagree with what most of the apostles taught (in Tertullian, Against Marcion, 4:3), that admission has a lot of significance in evaluating their credibility. It's also significant that Marcion accepted orthodox Christianity and the canonical version of Luke's gospel before later rejecting them (ibid., 1:1, 4:4). Tertullian claims that no church with a lineage from the apostles agreed with Marcion's view of God (ibid., 1:21), which is a major problem for the credibility of Marcionism at so early a date.
Other early rivals to traditional Christianity could be assessed in a similar manner. See, for example, the discussion of how Valentinianism was inferior to orthodoxy in Andreas Kostenberger and Michael Kruger, The Heresy Of Orthodoxy (Wheaton, Illinois: Crossway, 2010), 46. I've only provided several examples of orthodoxy's superiority, but these examples and others that could be cited have a wider application.