How do we know about Jesus' life and teachings?

Answer 1

1. We know about Jesus' life and teachings from the four gospels of the New Testament. They were all written anonymously and only attributed to the apostles whose names they now bear, later in the seocnd century, but there is no good reason to believe that those apostles were the actual authors.
2. Scholars say that the Gospels of Matthew and Luke were based in large part on the Gospel of Mark, with additional sayings material, including many teachings attributed to Jesus, taken from the hypothetical 'Q' document.
3. John's Gospel was inspired by Luke, with some material taken direct from Mark. The remarks of the early Church Father, Origen (Commentary on John), bear on how much this gospel adds to our knowledge of Jesus, "Although he does not always tell the truth literally, he always tells it spiritually."
4. Q appears to have been built up over time in four layers. It is somewhat problematic whether any of the teachings that come from Q were actually spoken by Jesus, except perhaps the first layer.
5. Raymond E. Brown (An Introduction to the New Testament) assumes that Mark seems to depend on traditions (and perhaps already shaped sources) received in Greek. We do not know what these sources were, but some modern scholars are developing credible hypotheses.

Answer 2

Anything we know about Jesus' life and teachings comes from the gospels. These were long thought to be accurate accounts, but that is no longer really the case.