On The Holy Trinity by St. Augustine
Chapter 14.--The Unobligated Death of Christ Has Freed Those Who Were Liable to Death.
18. What, then, is the righteousness by which the devil was conquered? What, except the righteousness of Jesus Christ? And how was he conquered? Because, when he found in Him nothing worthy of death, yet he slew Him. And certainly it is just, that we whom he held as debtors, should be dismissed free by believing in Him whom he slew without any debt. In this way it is that we are said to be justified in the blood of Christ. For so that innocent blood was shed for the remission of our sins. Whence He calls Himself in the Psalms, |Free among the dead.| For he only that is dead is free from the debt of death. Hence also in another psalm He says, |Then I restored that which I seized not;| meaning sin by the thing seized, because sin is laid hold of against what is lawful. Whence also He says, by the mouth of His own Flesh, as is read in the Gospel: |For the prince of this world cometh, and hath nothing in me,| that is, no sin; but |that the world may know,| He says, |that I do the commandment of the Father; arise, let us go hence.| And hence He proceeds to His passion, that He might pay for us debtors that which He Himself did not owe. Would then the devil be conquered by this most just right, if Christ had willed to deal with him by power, not by righteousness? But He held back what was possible to Him, in order that He might first do what was fitting. And hence it was necessary that He should be both man and God. For unless He had been man, He could not have been slain; unless He had been God, men would not have believed that He would not do what He could, but that He could not do what He would; nor should we have thought that righteousness was preferred by Him to power, but that He lacked power. But now He suffered for us things belonging to man, because He was man; but if He had been unwilling, it would have been in His power to not so to suffer, because He was also God. And righteousness was therefore made more acceptable in humility, because so great power as was in His Divinity, if He had been unwilling, would have been able not to suffer humility; and thus by Him who died, being thus powerful, both righteousness was commended, and power promised, to us, weak mortals. For He did one of these two things by dying, the other by rising again. For what is more righteous, than to come even to the death of the cross for righteousness? And what more powerful, than to rise from the dead, and to ascend into heaven with that very flesh in which He was slain? And therefore He conquered the devil first by righteousness, and afterwards by power: namely, by righteousness, because He had no sin, and was slain by him most unjustly; but by power, because having been dead He lived again, never afterwards to die. But He would have conquered the devil by power, even though He could not have been slain by him: although it belongs to a greater power to conquer death itself also by rising again, than to avoid it by living. But the reason is really a different one, why we are justified in the blood of Christ, when we are rescued from the power of the devil through the remission of sins: it pertains to this, that the devil is conquered by Christ by righteousness, not by power. For Christ was crucified, not through immortal power, but through the weakness which He took upon Him in mortal flesh; of which weakness nevertheless the apostle says, |that the weakness of God is stronger than men.|