How could Christ’s death be a sacrifice, since “those men who slew Christ did not perform any sacred act but rather wrought a great wrong”? (ST, III, 48, 3). Christ’s passion is no sacrificium but a maleficium.
Thomas answers by stressing the voluntary character of Christ’s death. Augustine said that “Christ offered Himself up for us in the Passion,” and Thomas adds that “this voluntary enduring of the Passion was most acceptable to God, as coming from charity.” For the killers, Christ’s death was indeed maleficium. But the cross was not primarily an execution. Most fundamentally, it was Christ’s own act, done out of eternal love: “Hence it is Christ who is said to have offered this sacrifice, and not the executioners.”