Luke 3:8

Bring forth therefore fruits worthy of repentance, and begin not to say within yourselves, We have Abraham to our father; for I say unto you that God is able of these stones to raise up children unto Abraham.

The usual response to any demand that the Israelites of that day should repent was to the effect that they did not need to repent, since they were sons of Abraham. The fact that all the promises were not to Abraham's fleshly descendants, but to his spiritual seed (the people who were of the character and faith of Abraham), was unknown to the Israel of that generation. Paul spelled it out in Romans; but here, the nigh impossible task of enlightenment fell on John the Baptist. He succeeded in such instances as John the apostle, and others who became followers of Jesus; but the majority of fleshly Israel only scoffed at the truth.