The set of books that we have in our Bible is called the “canon.” This word comes “from the Greek, and it originally meant “a measuring rod” or, as we might say, “a ruler.”” It was the standard to see if something was straight.  Books were admitted into The New Testament canon based on:
Apostolic authority—their association with Christ’s Apostles or those close to them,
Antiquity—whether the book was written during the time of the apostles,
Orthodoxy—if the book’s teachings were in line with the teaching of the apostolic writings know by the people who learned from the Apostle's and the churches which the Apostles founded,
Catholicity—if the book was accepted by many churches from many different places. 
A canon is the name given to a list of books believed to hold authority by the church. The Muratorian Canon, an early list of books that the church held as authoritative, was put together in 170 A. D. The Apostolic Canon and the Cheltenham Canon were listed in 300 and 360, respectfully. The Council of Hippo in 393 listed all 27 of the New Testament books, and the Council of Carthage in 410 listed all 27 as well.  A manuscript of the Muratorian Canon contains portions from all four gospels.
Several other types of writings show the church’s esteem of the gospel accounts. A gospel harmony is an attempt to place all of the sayings and events in the Gospels in chronological order, with accounts from each gospel of the same event side by side. A harmony was formed very early by “a Syrian Christian named Tatian.” It was put together in the late 100s. 
Many leaders in the early church also quoted Matthew, Mark, Luke and John. The four gospels were cited by Polycarp (c. 110-150 A. D.), Justin Martyr (c. 150-155), Clement of Alexandria (c. 150-215), Tertullian (c. 150-220), and Origen (c. 184-254). Irenaeus listed them as authoritative as early as 130-202. 
F. F. Bruce is clear, “It is evident that by A. D. 180 the idea of the fourfold Gospel had become so axiomatic throughout Christendom that it could be referred to as an established fact as obvious and inevitable and natural as the four cardinal points of the compass (as we call them) or the four winds”.
 Bruce L. Shelley. Church History in Plain Language, Second Edition (Dallas, Texas: Word Publishing 1995) 58.
 F. F. Bruce. The Cannon of Scripture (Downer’s Grove, Illinois: IVP Academic, 1998) 254-262.No line between footnotes.
 Norman L. Geisler. Systematic Theology Volume One: Introduction / Bible (Minneapolis, Minnesota: Bethany House, 2002), 535-536.
 J. Ed Komoszewski and M. James Sawyer and Daniel B. Wallace. Reinventing Jesus: How Contemporary Skeptics Miss the Real Jesus and Mislead Popular Culture (Grand Rapids, Michigan: Kregel Publications, 2006) 127.
 Craig L. Blomberg. The Historical Reliability of the Gospels, 28.
 Norman L. Geisler, Systematic Theology, 538.
 Josh McDowell. The DaVinci Code: A Quest for Answers (Holiday, Florida: Green Key Books, 2006), 26