There's a passage in Irenaeus that critics of Christianity have been abusing for a long time. It's relevant to the canon, particularly how reliable the early Christians were in making judgments about the origins and canonicity of the New Testament books:
"It is not possible that the Gospels can be either more or fewer in number than they are. For, since there are four zones of the world in which we live, and four principal winds, while the Church is scattered throughout all the world, and the pillar and ground of the Church is the Gospel and the spirit of life; it is fitting that she should have four pillars, breathing out immortality on every side, and vivifying men afresh. From which fact, it is evident that the Word, the Artificer of all, He that sits upon the cherubim, and contains all things, He who was manifested to men, has given us the Gospel under four aspects, but bound together by one Spirit." (Against Heresies, 3:11:8)
Here's how one skeptic treats the passage:
"Irenaeus of Lyon, writing against Marcion ca 190 CE was part of the scramble to create an authoritative canon to counter him and to define the faith. His dubious criteria for choosing just four gospels out of the dozens floating around at his time gave us the 'historical Jesus' as we know him. As he said, there can only be four gospels because there are four winds (directions of the compass), seems a bit tenuous as a means of weeding out other gospels. Why just four? 'Just because...' None of the four chosen can make a strong case for apostolicity." (Debunking Christianity)
There are a lot of problems with this skeptic's analysis, and I'm not going to be addressing all of the problems here. Did Irenaeus intend his comments in the manner often suggested by critics?
Anybody who has read much of Irenaeus should know that he often appeals to evidential concepts, such as eyewitness testimony and hostile corroboration. He does so in the passage just before the one under consideration. He refers to heretics' acceptance of the gospels as evidence of their validity (3:11:7). And in the section just after the one critics point to, Irenaeus argues for the four gospels, and against others, on the basis of internal evidence and what was historically handed down from the apostles (3:11:9).
Bruce Metzger comments (notice his use of the word "minor"):
"Among minor [canonical] criteria that the ancients sometimes applied was what may be called 'number-symbolism', of which we have conspicuous examples in Irenaeus and the Muratorian Canon....It is no doubt true that this use of numbers was more often a symbolical interpretation of the facts after the settlement of the different parts of the canon than as a means of determining that settlement." (The Canon Of The New Testament [New York: Oxford University Press, 1997], n. 5 on p. 254)
Everett Ferguson writes:
"It is often stated that Irenaeus was arguing for something new and had quite weak arguments for his position. This approach misunderstands the importance of number symbolism in the ancient world and Irenaeus's use of it. He does not argue for four gospels because there are four winds or four corners of the universe. He appeals to this symbolism because he has four gospels. If he had three, five, or some other number, he would have found an appropriately fitting analogy. Irenaeus does not see himself as an innovator, but as champion of a traditional position in the church over against the Marcionite narrowing and Valentinian expansion of the Gospel canon." (in Lee McDonald and James Sanders, edd., The Canon Debate [Peabody, Massachusetts: Hendrickson Publishers, 2002], p. 301)