Craig Keener has written a very nice article on the use of the Gospels – imagine that – to discover the Historical Jesus:
Scholars reconstruct the historical Jesus in various ways. Often they do so based on which sources about Jesus they privilege and how much they accept as reliable in these sources. Some scholars accept very little in the Gospels as reliable, hence sometimes offer reconstructions from the silence that remains – arguments from silence. Because of their minimal trust in the Gospels, others feel free to play some elements of the gospel tradition off against others – though usually these elements are not intrinsically contradictory.