Many factors should predispose us to take the Gospels as historically reliable sources for Jesus. First, they take the form of full-length ancient biographies at the apex of ancient biography’s historiographic focus, the early Roman empire. Also, they are written within living memory of their subject; today oral historians define the period of living memory as the period when those who knew the eyewitnesses remained alive, typically some 60-80 years.
Luke notes that others wrote about these events before him, thus some presumably while the witnesses remained alive; he also mentions consulting eyewitness sources himself and simply confirming information his audience already knew (Luke 1:1-4). The most influential witnesses, who were leaders of Jesus’s movement, were Jesus’s own disciples, trained by him (1 Cor. 9:5; 15:5; Gal. 2:9). A key responsibility of disciples was to pass on the teacher’s teachings, especially when they led a new movement founded by the teacher. In a society that already emphasized personal and collective memory, disciples of teachers were among the persons most intentional about preserving and passing on memories. While everyone forgets some things, the substance of key events can stick with us for years—all the more if we are regularly teaching about them to others.
Although the Gospels were written for an audience in the Diaspora (outside Judea and Galilee), they retain scores of elements grounded in events or even phrases known locally where the events occurred. They usually call the lake of Galilee a “sea”—a title given it only by locals. Jesus’s title “Son of Man” is a purely Semitic expression that makes little sense in Greek. Jesus’s forms of parables, and even his introductory formulas for them, fit those of contemporary Jewish sages. His debates with Pharisees over issues such as pure vessels, divorce, and the greatest commandment reflect known Pharisaic debates from his generation. Archaeology attests Capernaum’s synagogue and other sites unknown to Diaspora Christians; indeed, although Bethsaida was renamed Julias in the year 30, the Gospels always retain the earlier name by which the city was known during Jesus’s ministry. These and many other traits would not and could not have been invented by later Diaspora Christians, but instead preserve authentic memories from the earliest witnesses.
Not everyone defines “reliable” the same way, of course. The Gospels clearly have differences. No one expected all biographies to arrange every event in chronological order. Paraphrase was expected practice. Different writers approach subjects from different angles, though these may be complementary. But it seems disingenuous to appeal to such differences, which are fairly small by the standards of ancient literature, to undercut the strong body of evidence supporting the substance of the Gospel accounts as reflecting genuine information.
Craig S. Keener, PhD. is the F. M. and Ada Thompson Professor of Biblical Studies at Asbury Theological Seminary.
For Professor Keener's full treatment of this question:
Craig S. Keener, Christobiography: Memories, History, and the Reliability of the Gospels. Grand Rapids: Eerdmans, 2019.
Craig S. Keener, The Historical Jesus of the Gospels. Grand Rapids: Eerdmans, 2009.