That Paul's letters are thought to be older than any of the Gospel accounts by Matthew, Mark, Luke, and John? Mark's Gospel, which is now accepted as the earliest one, dates from AD 70—just before the fall of Jerusalem. Paul's letters were written from AD 50—68, possibly as early as AD 48, which puts at least some of them much closer to the life and times of Jesus and the disciples.
Why is this important? Because Paul's comments about Jesus and remarks on decisions by the early Christian Church are a bit like hearing from an eyewitness as compared to the child or grandchild of the eyewitness. In essence, the words and ideas aren't so shaded by memory or a built-up tradition, and relay the original event without the accretions of history.
As F. F. Bruce explains in an article on Paul's letters, "... when the earlier Pauline Epistles mention a saying or an action of Jesus, that mention is the oldest written account that we have, antedating the earliest Gospel record by several years. ... Since our Gospels stand at the beginning of the [New Testament], and deal with a period of time some years earlier than Paul's conversion and apostolic activity, we tend to forget that even the earliest of the four was not composed until Paul's career was wellnigh finished." Peake's Commentary on the Bible (Nashville, Tennessee: Thomas Nelson Publishers, 1986). p. 927 .
