Thomas Aquinas, Compendium of Theology

Since the human race was freed by Christ from the evils flowing from the sin of our first parent, it was fitting that, as He bore our ills to free us from them, the first fruits of man's restoration effected by Him should make their appearance in Him. This was done that Christ might be held up to us as a sign of salvation in two ways. First, we learn from His passion what we brought down on ourselves by sin and what suffering had to be undergone for us to free us from sin. Secondly, we see in His exaltation what is proposed to us to hope for through Him.

In triumph over death, which resulted from our first parent's sin, Christ was the first of all men to rise to immortal life. Thus, as life first became mortal through Adam's sin, immortal life made its first appearance in Christ through the atonement for sin He offered. Others, it is true, raised up either by Christ or by the prophets, had returned to life before Him; yet they had to die a second time. But "Christ rising again from the dead, dieth now no more" (Rom. 6:9). As He was the first to escape the necessity of dying, He is called "the first begotten of the dead" (Apoc. 1:5) and "the first fruits of them that sleep" (I Cor. 15:20). Having thrown off the yoke of death, He was the first to rise from the sleep of death.

Christ's resurrection was not to be long delayed, nor, on the other hand, was it to take place immediately after death. If He had returned to life immediately after death, the fact of His death would not have been well established; and if the resurrection had been long delayed, the sign of vanquished death would not have appeared in Him, and men would not have been given the hope that they would be rescued from death by Him. Therefore He put off the resurrection until the third day, for this interval was judged sufficient to establish the truth of His death, and was not too long to wither away the hope of liberation. If it had been delayed for a longer time, the hope of the faithful might have begun to suffer doubt. Indeed, on the third day, as though hope were already running out, some were saying: "We hoped that it was He that should have redeemed Israel" (Luke 24:21).

However, Christ did not remain dead for three full days. He is said to have been in the heart of the earth for three days and three nights, according to that figure of speech whereby a part is often taken for the whole. For, since one natural day is made up of a day and a night, Christ is said to have been dead during the whole of any part of a day or a night that is counted while He was lying in death.

Moreover, in the usual practice of Scripture, night is figured in with the following day, because the Hebrews reckon time by the course of the moon, which begins to shine in the evening. Christ was in the sepulcher during the latter part of the sixth day, and if this is counted along with the preceding night, it will be more or less one natural day. He reposed in the tomb during the night following the sixth day, together with the whole of the Sabbath day, and so we have two days. He lay dead also during the next night, which preceded the Lord's Day, on which He rose, and this occurred either at midnight, according to Gregory, or at dawn, as others think. Therefore, if either the whole night, or a part of it together with the Lord's Day following, is taken into our calculation, we shall have the third natural day.

The fact that Christ wished to rise on the third day is not without mysterious significance; for so He was able to show that He rose by the power of the whole Trinity. Sometimes the Father is said to have raised Him up, and sometimes Christ Himself is said to have risen by His own power. These two statements do not contradict each other, for the divine power of the Father is identical with that of the Son and of the Holy Spirit. Another purpose was to show that the restoration of life was accomplished, not on the first day of the world, that is, under the natural law, nor on the second day, that is, under the Mosaic law, but on the third day, that is, in the era of grace.

The fact that Christ lay in the sepulcher for one whole day and two whole nights also has its meaning: by the one ancient debt Christ took on Himself, that of punishment, He blotted out our two ancient debts, sin and punishment, which are represented by the two nights.