The New Testament contains the earliest sources for information about Jesus of Nazareth. Some of the material, such as Paul’s letters, was written as early as AD 48 or 49 and shortly thereafter (Galatians, 1-2 Thessalonians, and 1-2 Corinthians).
The Gospels began to circulate sometime in AD 60s. Virtually all scholars agree that the New Testament contains material that goes back directly to Jesus of Nazareth. A significant amount of scholarly activity is focused on isolating the sub-strata of material that forms the basis the Gospel narratives, including eyewitness accounts, oral traditions, and even written sources. No other sources from the first or second century AD provide material that can be used with the confidence that the New Testament provides in reconstructing the life and ministry of Jesus Christ.
“This means that if historians want to know what Jesus said and did they are more or less constrained to use the New Testament Gospels as their principal sources. Let me emphasize that this is not for religious or theological reasons-for instance, that these and these alone can be trusted.
"It is for historical reasons, pure and simple. Jesus Christ is scarcely mentioned by non-Christian sources for over a century after His death, and the other authors of the New Testament are more concerned with other matters.
"Moreover, the Gospel accounts outside the New Testament tend to be late and legendary, of considerable interest in and of themselves, but of little use to the historian interested in knowing what happened during Jesus’ lifetime. . . . the only real sources available to the historian interested for the life of Jesus are therefore the New Testament Gospels.”
--Bart D. Ehrman is James A. Gray Distinguished Professor of Religious Studies at the University of North Carolina at Chapel Hill.