According to RationalWiki:
The Gospels — the most direct and contemporaneous biographical material of Jesus known — were…picked out of a much greater number of books by a Catholic bishop named Eusebius around 250 years later.
But that’s a ham-fisted effort by RW. A much more nuanced understanding of canonicity comes from Richard Bauckham:
…The term canon means that there are four and only four authentic and authoritative Gospels. We find an unequivocal expression of that latter view for the first time in Irenaeus of Lyons, the great theologian who was writing around the end of the second century. We find Irenaeus defending the view that there are four authentic Gospels, no fewer and no more, against proposals that the church should only use one Gospel as well as against advocates of many Gospels besides the four…
So how did the four Gospel canon come about? The first thing to say is that we have to admit that there’s a great deal we don’t know. I think the reason for that is that the actual process of canonization, the discernment and decision about the status of various Gospel writings, was primarily a grassroots process. It took place at the level of local Christian communities. Some such process had to happen in local Christian communities at a very early stage, because, from an early stage, in Christian worship, there were readings from the Old Testament scriptures and also from Christian writings, and no doubt there was often exposition of both. Any church had to decide which Christian writings were suitable for reading alongside the Old Testament, which must have meant pretty much scriptural status. We can imagine a process by which a degree of consensus would have spread, as one community heard what other communities were reading as scripture. An informal consensus would have grown up, at least about some groups, though it will never have been a complete consensus. But the fact is that we have limited evidence of how that happened.
- RationalWiki (2013). RationalWiki Atheism FAQ for the Newly Deconverted. Available http://rationalwiki.org/wiki/RationalWiki_Atheism_FAQ_for_the_Newly_Deconverted. Last accessed 31st Dec 2013.
- Bauckham, R. (n.d.) The Canonicity of the Four Gospels [sermon]. Available http://richardbauckham.co.uk/uploads/Sermons/Canonicity%20of%20the%20Gospels.pdf. Last accessed 8th Jun 2015.