John does not seem to be running a seeker friendly church here! What kind of preacher calls those who come to be baptized a brood of vipers? But John was not content to gain a large number of followers, especially those concerned only with superficial righteousness. He calls out the crowd for wanting to attain one more spiritual sticker on their chart. John's listeners needed to know and take seriously the divine judgment to come.
John warns them that if they are going to go through the ritual of baptism, they needed to be willing to live a life of true repentance. No religious designation or even being considered children of Abraham and members of the chosen nation would save them. God was working in the world to remove those claiming to bear his name who did not bear fruit and graft in those who would enter into righteousness through Christ.
Even today many take pride in religious designation. You may be certain that your denomination is better than all the others and secretly you believe that you will get some bonus "fruit points" for being a member (see Baptists in heaven joke for some levity on the subject). Perhaps you can trace your Christian lineage back decades, centuries, or even back to the apostles themselves.
None of this will save you. Only true repentance and belief in Jesus as Savior can accomplish this. And if you truly repent and truly believe on Him, your life will bear good fruits of repentance. If others cannot look at your life and see good deeds, if you are not daily crucifying the old self with its desires, if you do not look more Christlike as each year passes then you should examine your heart and mind to discern if you have truly repented and believed.
But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control; against such things there is no law. - Galatians 5:22-23