Cast The First Stone Meaning

(idiomatic) To act self-righteously in accusing another person, believing that one is blameless.

Example: 2007, March 10, Mr Gingrich, on the impeachment proceedings in 1999. Reported in The Guardian
  I knew I couldn't cast the first stone as I knew I had weaknesses. ... (But) as a leader of the government trying to uphold the rule of law I have no choice but to move forward and say you cannot accept perjury in your highest officials.