Why did Thomas Becket oppose punishing clergy twice for the same crime?

Thomas Becket’s opposition to the secular punishment of "criminous clerks" was rooted in the principle of ne bis in idem—the idea that a person should not be punished twice for the same fault.1

Becket argued that because a cleric could already be tried, degraded, or exiled by an ecclesiastical court, subjecting the same individual to further punishment by a secular authority constituted a violation of canon law. While King Henry II contended that the leniency of church courts encouraged clerical crime, Becket maintained that the church held the sole right to discipline its own members.12

This dispute became a central point of contention in the Constitutions of Clarendon, which sought to restrict these ecclesiastical privileges and allow secular courts to punish priests charged with serious felonies. Becket’s rigid stance on this issue was canonically arguable, and his eventual martyrdom forced the king to moderate his efforts to curb the power of the church courts.21