One of the most important aspects of historical apologetics for the person of
Jesus, for the reliability of the gospels, is source criticism of the gospels. The
modern New Testament scholar doesn't simply quote from the Gospel of Luke or
the Gospel of John; rather, he tries to discern the sources upon which the
evangelists drew in writing their gospels. Luke,
for example, refers to the many people who have tried to write an account of
the things that have transpired concerning Jesus of Nazareth, and he says
that having consulted these he will now attempt to give an orderly account of
his own. The other gospel writers also draw upon earlier traditions, whether
written or more likely oral traditions, that come to be embodied in their
gospels. This is very helpful to the task of Christian evidences because even
though Matthew and Luke probably drew upon Mark and therefore are not
independent of Mark, nevertheless the sources behind Luke and Matthew and the
sources behind Mark may well be independent. For example, in addition to
Mark most scholars think that Luke and Matthew drew upon another source that is
arbitrarily called the Q document, which was a source of sayings of Jesus,
teachings of Jesus, rather than an account of the events of his life, and
they blended the the material from this source with Mark's Gospel in writing
their own gospels. So if you can show that something is attested both in Mark
and in Q, then you're dealing with independent
sources; you're not just counting the gospels, you're looking at the sources
they drew upon. Moreover, some of the gospels are independent of other gospels.
For example, Luke and Matthew and Mark are normally thought to be independent
of John. This is significant because when you find in the synoptic gospels, Matthew,
Mark, and Luke, an account of the triumphal entry of Jesus into Jerusalem,
and you also find an account of the triumphal entry in John, you have
independent accounts of the same event, and this can help to increase the
historian's confidence in the reliability of this story. Paul then provides
additional independent information about Jesus. The letters of Paul have been
called a fifth gospel. In Paul's letters you can read about Jesus' life, his
teaching, his miracles, his last supper, his betrayal, his crucifixion, his burial,
his resurrection from the dead, and this is an independent source from the four
gospels for these events in the life of Jesus of Nazareth, and therefore are very
valuable, especially since Paul's information is so early, so soon after
the events took place. So that's what's meant by independent sources, it's not
just talking about the gospels but it's dealing with the sources upon which the
gospels and Paul drew which are independent of one another.