Ancient Near-Eastern society was group-oriented (not individualistic) and agonistic, meaning they were constantly competing socially for honor (much like Japanese culture). A person was either ascribed honor through inheritance (bloodline) or acquired it through challenge and riposte.
Honor in general was taken very seriously, about as serious as we in the west treat money, if not more so. Among the two types of honor (ascribed and acquired), ascribed honor was defended the most vigorously.
One way to attempt to discredit someone's ascribed honor was to question their familial lineage. Conversely, one could defend their ascribed honor by providing a genealogy (hence Matthew 1 and Luke 3).
"Given this emphasis on defensive [marriage] strategy and holy seed, imputations of doubtful lineage are among the gravest insults in the culture, sure to get prompt attention. For example, John the Baptist's calling the Pharisees and Sadducees of Jerusalem a "brood of vipers" means nothing less than "snake bastards," a doubly offensive term... These are powerful insults in a culture where purity of lineage is a central concern." [New Testament World, 154]
After calling the Pharisees and Sadducees "snake bastards", John the Baptist "... anticipates that the crowd will respond with assertions of proper lineage, and consequently poses the alternative view that the true lineage has a moral rather than simply a biological base." [Social Science Commentary on the Synoptic Gospels, 33]. He did this by claiming, "Do not presume to say to yourselves, 'We have Abraham as our ancestor'; for I tell you, God is able from these stones to raise up children of Abraham."
Jesus likewise used such insults against His opponents: "brood of vipers" in Matthew 12:34, "adulterous generation" in 12:39 (or "generation of bastards"), and so forth. Remember also when Jesus went to teach in Nazareth, the people questioned His authority because they knew of His humble familial origins:
"Isn't this the carpenter's son? Isn't His mother called Mary, and His brothers James, Joseph, Simon, and Judas? And His sisters, aren't they all with us? So where does He get all these things?" And they were offended by Him." - Matt. 13:55-56
These terms used by John, Jesus, and their opponents where powerful social rhetorical devices used to slander their honor in the deepest way possible. Most of us in the west cannot fully understand just how deep these words were meant to cut. Kind of takes the soft edge off our Lord that we often like to ascribe, doesn't it?
