In what were the Jewish religious leaders basing their sense of security in and how does John’s message bring a powerful rebuke to such notions?

“And do not presume to say to yourselves, 'We have Abraham as our father,' for I tell you, God is able from these stones to raise up children for Abraham. Even now the axe is laid to the root of the trees. Every tree therefore that does not bear good fruit is cut down and thrown into the fire.” (Matt 3:9-10).

The Pharisees and Sadducees were experts in the Law and genealogies (hence the importance of the genealogical introduction of Matthew's gospel). God's chosen people were the Jews and only the Jews who could trace their ancestry to Abraham (the Samaritans were ostracized for being unable to). The Jews understood that they were born into God's favor as part of the nation of Israel, and could only fall from that favor by failing to follow God's commandments in the Law of Moses.   Thus, the Pharisees and Sadducees' sense of security was derived from their personal efforts in following the Law of Moses and national security since “We have Abraham as our father.”  In a way, the religious leaders believed that for God to make good on His promises to Abraham He needed the nation of Israel and by extension those who kept the law of Moses.

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 This is why “John uses the strongest language to jar them out of their false sense of personal and national security” (Elwell, 727), by telling them judgment loomed for those who followed the letter of the law, but not the heart of the law. In fact, John warns that being a child of Abraham is irrelevant if they don't share Abraham's character (Elwell, 809). For this reason, “John, and Jesus after him, targets the reconstruction of the interior life not its external scaffolding” (Howell, 26) emphasizing the necessity for a change of mind and a change of life over religious achievement. Moreover, the religious authorities needed to truly repent since “God can raise up children for Abraham” without the nation of Israel. In fact, God's Kingdom and the salvation it bestows was about to be proclaimed to all people regardless of social standing, ethnic heritage or good works (Howell, 26).

• Elwell, Walter. A.. Evangelical Commentary on the Bible. Grand Rapids: Baker Book House. 1989.
  
• Howell Jr., Don N.. The Passion of the Servant. Eugene: Wipf & Stock - Resource Publications. 2009.