To cast the first stone
Be the first to attack a sinner. The implication in Jesus' teaching was that the members of the congregation were only in a position to condemn a sinner if they were without sin themselves - in other words, 'judge not lest you be judged'.
Casting the first stone was adroitly satirised in the stoning scene in The Life of Brian.
This phrase derives from the Bible- John 8:7. The earliest biblical example of it in print in English comes in Miles Coverdale's Bible, 1535:
Now whyle they contynued axynge him, he lift him self vp, and sayde vnto them: He that is amonge you without synne, let him cast the first stone at her.