Objection 1. It would seem that Christ did not have a true body after His Resurrection. For a true body cannot be in the same place at the same time with another body. But after the Resurrection Christ's body was with another at the same time in the same place: since He entered among the disciples "the doors being shut," as is related in John 20:26. Therefore it seems that Christ did not have a true body after His Resurrection.
Objection 2. Further, a true body does not vanish from the beholder's sight unless perchance it be corrupted. But Christ's body "vanished out of the sight" of the disciples as they gazed upon Him, as is related in Luke 24:31. Therefore, it seems that Christ did not have a true body after His Resurrection.
Objection 3. Further, every true body has its determinate shape. But Christ's body appeared before the disciples "in another shape," as is evident from Mark 16:12. Therefore it seems that Christ did not possess a true body after His Resurrection.
On the contrary, It is written (Luke 24:37) that when Christ appeared to His disciples "they being troubled and frightened, supposed that they saw a spirit," as if He had not a true but an imaginary body: but to remove their fears He presently added: "Handle and see, for a spirit hath not flesh and bones, as you see Me to have." Consequently, He had not an imaginary but a true body.
I answer that, As Damascene says (De Fide Orth. iv): that is said to rise, which fell. But Christ's body fell by death; namely, inasmuch as the soul which was its formal perfection was separated from it. Hence, in order for it to be a true resurrection, it was necessary for the same body of Christ to be once more united with the same soul. And since the truth of the body's nature is from its form it follows that Christ's body after His Resurrection was a true body, and of the same nature as it was before. But had His been an imaginary body, then His Resurrection would not have been true, but apparent.
Reply to Objection 1. Christ's body after His Resurrection, not by miracle but from its glorified condition, as some say, entered in among the disciples while the doors were shut, thus existing with another body in the same place. But whether a glorified body can have this from some hidden property, so as to be with another body at the same time in the same place, will be discussed later (XP, 83, 4) when the common resurrection will be dealt with. For the present let it suffice to say that it was not from any property within the body, but by virtue of the Godhead united to it, that this body, although a true one, entered in among the disciples while the doors were shut. Accordingly Augustine says in a sermon for Easter (ccxlvii) that some men argue in this fashion: "If it were a body; if what rose from the sepulchre were what hung upon the tree, how could it enter through closed doors?" And he answers: "If you understand how, it is no miracle: where reason fails, faith abounds." And (Tract. cxxi super Joan.) he says: "Closed doors were no obstacle to the substance of a Body wherein was the Godhead; for truly He could enter in by doors not open, in whose Birth His Mother's virginity remained inviolate." And Gregory says the same in a homily for the octave of Easter (xxvi in Evang.).
Reply to Objection 2. As stated above (Question 53, Article 3), Christ rose to the immortal life of glory. But such is the disposition of a glorified body that it is spiritual, i.e. subject to the spirit, as the Apostle says (1 Corinthians 15:44). Now in order for the body to be entirely subject to the spirit, it is necessary for the body's every action to be subject to the will of the spirit. Again, that an object be seen is due to the action of the visible object upon the sight, as the Philosopher shows (De Anima ii). Consequently, whoever has a glorified body has it in his power to be seen when he so wishes, and not to be seen when he does not wish it. Moreover Christ had this not only from the condition of His glorified body, but also from the power of His Godhead, by which power it may happen that even bodies not glorified are miraculously unseen: as was by a miracle bestowed on the blessed Bartholomew, that "if he wished he could be seen, and not be seen if he did not wish it" [Apocryphal Historia Apost. viii, 2]. Christ, then, is said to have vanished from the eyes of the disciples, not as though He were corrupted or dissolved into invisible elements; but because He ceased, of His own will, to be seen by them, either while He was present or while He was departing by the gift of agility.
Reply to Objection 3. As Severianus [Peter Chrysologus: Serm. lxxxii] says in a sermon for Easter: "Let no one suppose that Christ changed His features at the Resurrection." This is to be understood of the outline of His members; since there was nothing out of keeping or deformed in the body of Christ which was conceived of the Holy Ghost, that had to be righted at the Resurrection. Nevertheless He received the glory of clarity in the Resurrection: accordingly the same writer adds: "but the semblance is changed, when, ceasing to be mortal, it becomes immortal; so that it acquired the glory of countenance, without losing the substance of the countenance." Yet He did not come to those disciples in glorified appearance; but, as it lay in His power for His body to be seen or not, so it was within His power to present to the eyes of the beholders His form either glorified or not glorified, or partly glorified and partly not, or in any fashion whatsoever. Still it requires but a slight difference for anyone to seem to appear another shape.
Source: Summa Theologica (New Advent)