Penal substitution (sometimes, esp. in older writings, called forensic theory) is a theory of the atonement within Christian theology, which declares that Christ, voluntarily submitting to God the Father's plan, was punished (penalized) in the place of sinners (substitution), thus satisfying the demands of justice so God can justly forgive sins making us at one with God (atonement). It began with Luther and continued to develop with the Calvinist tradition as a specific understanding of substitutionary atonement, where the substitutionary nature of Jesus' death is understood in the sense of a substitutionary fulfilment of legal demands for the offenses of sins.