Question: When did the church officially decide on the four Gospels?
Bart Ehrman—James A. Gray Distinguished Professor of Religious Studies at the University of North Carolina, author of three NY Times bestsellers, and one of the most active promoters of gnosticism in the modern era—gives an answer that is popular today, but it is just not true.
Ehrman claims that Irenaeus, the acclaimed missionary and bishop from Lyons, Gaul, sifted the Gospels down from 30 to 4 in the late second century. He complains that Irenaeus' argument for rejecting 26 gospels is unconvincing ...
The problem with Ehrman's complaint is that Irenaeus was not arguing for reducing the number of Gospels from 30 to 4. The Gospels were already 4 in his time, and he was simply arguing that this was an appropriate number, matching the 4 corners of the earth and the 4 winds of heaven.
Irenaeus wrote Against Heresies, which Ehrman was citing, around A.D. 185. The Muratorian Fragment dates from about 20 years earlier, and it lists the books accepted by the churches. It lists the four Gospels we know and no others.
Justin Martyr, too, from about 30 years before Irenaeus, mentions "the apostles' memoirs." He does not directly state that there are only four, but he quotes only the four we know. Further, a decade later, a disciple of Justin's named Tatian produced the Diatessaron, a harmony of the four Gospels we know.
Earlier writers quote the four Gospels often, but never any other except one quote from the Gospel of Thomas which is found in an early Christian sermon known as 2 Clement. There is some question whether that quote is original to the Gospel of Thomas anyway. Clement of Alexandria cites that same single verse, just before the end of the second century, as being from a work called The Gospel of the Egyptians.
So when did the Church officially decide upon the four Gospels?
We simply find the "memoirs of the apostles" being quoted early. They are called Gospels, and around A.D. 161, the Muratorian Fragment, lists them as among the books and letters that the churches accept.
Later lists of "the canon," or the accepted books of the church, also reference those four Gospels.
In the 16th century, the Roman Catholic Church finally held a council to officially decide upon the books that constituted the Bible. The council was the Council of Trent, and no other council with authority has ever issued an official canon (Catholic Encyclopedia). Even the Council of Trent has authority only for the Roman Catholic Church, not for Protestant or Orthodox churches.
It's often claimed that the Synod of Hippo in A.D. 393 settled the canon (and thus the four Gospels), but this can't be true, either. Augustine (the famous St. Augustine) become the bishop of Hippo just 3 years after the synod of Hippo. Yet Augustine, while he was the bishop of Hippo, wrote:
Clearly, Augustine, the bishop of Hippo, didn't know that the Synod of Hippo had set the canon for the whole church!
Thus, the earliest we can claim that the church "officially" set the canon was at the Council of Trent between 1546 and 1563. Unofficially, though, based on quotes and references in writings of the early second century, it is clear that the churches were using the four Gospels, and the four Gospels only, by the beginning of the second century.