But the devil had to be overcome not by the power of God but by his justice. For what is more powerful than the omnipotent? What creature’s power can be compared with the power of the creator? Through the fault of his own turning away the devil became a lover of power and a deserter and opponent of justice; and people imitate him more fully the more they ignore or even detest justice and set their hearts on power, consumed either by delight in its acquisition or by desire for its possession. So in rescuing humankind from the power of the devil God chose that the devil should be conquered not by power but by justice, so that people too in imitation of Christ should seek to conquer the devil by justice and not by power. It is not that power had to be avoided as if it were something evil, but that the priority over power needed to be preserved. . . .
What then is that justice whereby the devil was conquered? What was it but the justice of Jesus Christ? And how was the devil conquered? Because, although he found nothing worthy of death in Jesus, he killed him. So it is unquestionably just that the debtors whom he was holding should be allowed to go free on the strength of their believing in the one whom he killed despite his having no debt. That is what is meant when we are said to be “justified by the blood of Christ.” Thus was his innocent blood shed for the remission of our sins. . . .
The conquest of the devil would surely not have been achieved in so perfectly just a manner had Christ chosen to deal with him by power instead of justice. But in fact he put power second and obligation first. . . . So he conquered the devil first by justice and then by power—by justice because he had no sin and was most unjustly killed by him; by power because “being dead he came to life again never to die any more.” . . . The devil was conquered at the very moment when he thought he was the conqueror—namely, when Christ was killed. Then it was that his blood—the blood of him who had no sin at all—was shed for the remission of our sins. Its purpose was that those whom the devil was justifiably holding bound in a condition of death since they were guilty of sin should be justifiably released by him on whom the devil had unjustifiably inflicted the penalty of death when he was guilty of no sin. By this justice he was conquered.
On the Trinity 13.12.17–15.19.