Thomas Aquinas, Compendium of Theology

Christ wished to suffer not only death, but also the other ills that flow from the sin of the first parent to his posterity, so that, bearing in its entirety the penalty of sin, He might perfectly free us from sin by offering satisfaction. Of these ills, some precede death, others follow death. Prior to the death of the body come natural sufferings, such as hunger, thirst, and weariness, and also sufferings inflicted by violence, such as wounding, scourging, and the like. Christ wished to endure all these sufferings, since they stem from sin. If man had not sinned, he would not have experienced the affliction of hunger or of thirst or of fatigue or of cold, and he would not have had to undergo the suffering caused by external violence.

Christ bore these sufferings for a different reason from that on account of which other men endure them. In other men there is nothing that can resist these sufferings. But Christ had at His disposal means to withstand evils of this sort: not only the uncreated power of His divinity, but also the beatitude of His soul, which is so powerful that, as Augustine says, its happiness in its own way flows over into the body. Thus after the resurrection, by the very fact that the soul will be glorified by the vision of God in unrestricted and full fruition, the body united to the glorified soul will be rendered glorious, impassible, and immortal. Therefore, since the soul of Christ enjoyed the vision of God in the highest degree of perfection, His body should in consequence, so far as the power of this vision is concerned, have been rendered impassible and immortal by an overflowing of glory from the soul to the body. But divine wisdom so disposed matters that Christ's body would suffer at the very time His soul was enjoying the vision of God, with no overflow of glory from the soul to the body. For, as we have said, all that was physical in Christ's human nature was subject to His will. Hence at His good pleasure He could prevent natural redundance from His higher to His lower parts, and so could allow any part to suffer or do whatever would be proper to it without interference from any other part. This, of course, is impossible in other men.

This also accounts for the fact that during His passion Christ suffered most excruciating pain of body. For His bodily pain was in no way lessened by the higher joy of His rational soul, just as, conversely, pain of body did not obstruct the joy of His rational soul.

This reveals, too, that Christ alone was both a viator and a comprehensor. He enjoyed the vision of God, which characterizes the comprehensor, but in such a way that His body remained subject to sufferings, which characterizes the wayfarer. And since a wayfarer has power to merit, either for himself or for others, by the good works he performs from the motive of charity, Christ too, although He was a comprehensor, merited both for Himself and for others by His works and sufferings.

For Himself Christ merited, not indeed glory of soul, which He had from the first instant of His conception, but glory of body, which He won by suffering. For us, too, each of His sufferings and actions was profitable unto salvation, not only by way of example, but also by way of merit; owing to the abundance of His charity and grace, He could merit grace for us, so that thus the members might receive of the fullness of the head.

Any suffering of His, however slight, was enough to redeem the human race, if the dignity of the sufferer is considered. For the more exalted the person on whom suffering is inflicted, the greater is the injury judged to be; for instance, a greater outrage is committed if one strikes a prince than if one strikes a common man of the people. Consequently, since Christ is a person of infinite dignity, any suffering of His has an infinite value, and so suffices for the atonement of infinitely many sins. Yet the redemption of the human race was accomplished, not by this or that slight suffering, but by Christ's death, which, for reasons listed above, He chose to endure to redeem the human race from its sins. For in any purchasing transaction there is required, not only a stipulated amount of appreciable commodity, but also the application of the price to the purchase.