Scribes and Pharisees bring a woman they say was caught in adultery and ask Jesus what should be done because Moses commanded such offenders to be stoned. The narrator adds that the question is a test intended to create grounds for accusation.
The scene invokes real biblical law but does so in a visibly problematic prosecution.
Torah condemns adultery and treats both participants as accountable. Deuteronomy also requires adequate witnesses for capital cases and says that the witnesses' hands are to be first in carrying out the sentence. Only the woman is brought before Jesus, although adultery necessarily involved another party. The passage does not explain where the man is.
Jesus' challenge, 'He that is without sin among you, let him first cast a stone,' therefore occurs within a setting already concerned with witnesses and their responsibility, not as a general declaration that only morally perfect people may ever judge wrongdoing.
The legal background keeps the famous saying from becoming a slogan against every form of moral judgment. Jesus turns the responsibilities of accusers and witnesses back upon the men attempting to use the woman as a trap.
The accusers depart, but Jesus does not redefine adultery as morally irrelevant; his final command is, 'Go, and sin no more.'