Objection 1. It would seem that Christ's body was reduced to dust in the tomb. For just as man dies in punishment of his first parent's sin, so also does he return to dust, since it was said to the first man after his sin: "Dust thou art, and into dust thou shalt return" (Genesis 3:19). But Christ endured death in order to deliver us from death. Therefore His body ought to be made to return to dust, so as to free us from the same penalty.
Objection 2. Further, Christ's body was of the same nature as ours. But directly after death our bodies begin to dissolve into dust, and are disposed towards putrefaction, because when the natural heat departs, there supervenes heat from without which causes corruption. Therefore it seems that the same thing happened to Christ's body.
Objection 3. Further, as stated above (Article 1), Christ willed to be buried in order to furnish men with the hope of rising likewise from the grave. Consequently, He sought likewise to return to dust so as to give to them who have returned to dust the hope of rising from the dust.
On the contrary, It is written (Psalm 15:10): "Nor wilt Thou suffer Thy holy one to see corruption": and Damascene (De Fide Orth. iii) expounds this of the corruption which comes of dissolving into elements.
I answer that, It was not fitting for Christ's body to putrefy, or in any way be reduced to dust, since the putrefaction of any body comes of that body's infirmity of nature, which can no longer hold the body together. But as was said above (50, 1, ad 2), Christ's death ought not to come from weakness of nature, lest it might not be believed to be voluntary: and therefore He willed to die, not from sickness, but from suffering inflicted on Him, to which He gave Himself up willingly. And therefore, lest His death might be ascribed to infirmity of nature, Christ did not wish His body to putrefy in any way or dissolve no matter how; but for the manifestation of His Divine power He willed that His body should continue incorrupt. Hence Chrysostom says (Cont. Jud. et Gent. quod 'Christus sit Deus') that "with other men, especially with such as have wrought strenuously, their deeds shine forth in their lifetime; but as soon as they die, their deeds go with them. But it is quite the contrary with Christ: because previous to the cross all is sadness and weakness, but as soon as He is crucified, everything comes to light, in order that you may learn it was not an ordinary man that was crucified."
Reply to Objection 1. Since Christ was not subject to sin, neither was He prone to die or to return to dust. Yet of His own will He endured death for our salvation, for the reasons alleged above (Question 51, Article 1). But had His body putrefied or dissolved, this fact would have been detrimental to man's salvation, for it would not have seemed credible that the Divine power was in Him. Hence it is on His behalf that it is written (Psalm 29:10): "What profit is there in my blood, whilst I go down to corruption?" as if He were to say: "If My body corrupt, the profit of the blood shed will be lost."
Reply to Objection 2. Christ's body was a subject of corruption according to the condition of its passible nature, but not as to the deserving cause of putrefaction, which is sin: but the Divine power preserved Christ's body from putrefying, just as it raised it up from death.
Reply to Objection 3. Christ rose from the tomb by Divine power, which is not narrowed within bounds. Consequently, His rising from the grave was a sufficient argument to prove that men are to be raised up by Divine power, not only from their graves, but also from any dust whatever.
Source: Summa Theologica (New Advent)