Objection 1. It would seem that Christ's body ought not to have risen with its scars. For it is written (1 Corinthians 15:52): "The dead shall rise incorrupt." But scars and wounds imply corruption and defect. Therefore it was not fitting for Christ, the author of the resurrection, to rise again with scars.
Objection 2. Further, Christ's body rose entire, as stated above (Article 3). But open scars are opposed to bodily integrity, since they interfere with the continuity of the tissue. It does not therefore seem fitting for the open wounds to remain in Christ's body; although the traces of the wounds might remain, which would satisfy the beholder; thus it was that Thomas believed, to whom it was said: "Because thou hast seen Me, Thomas, thou hast believed" (John 20:29).
Objection 3. Further, Damascene says (De Fide Orth. iv) that "some things are truly said of Christ after the Resurrection, which He did not have from nature but from special dispensation, such as the scars, in order to make it sure that it was the body which had suffered that rose again." Now when the cause ceases, the effect ceases. Therefore it seems that when the disciples were assured of the Resurrection, He bore the scars no longer. But it ill became the unchangeableness of His glory that He should assume anything which was not to remain in Him for ever. Consequently, it seems that He ought not at His Resurrection to have resumed a body with scars.
On the contrary, Our Lord said to Thomas (John 20:27): "Put in thy finger hither, and see My hands; and bring hither thy hand, and put it into My side, and be not faithless but believing."
I answer that, It was fitting for Christ's soul at His Resurrection to resume the body with its scars. In the first place, for Christ's own glory. For Bede says on Luke 24:40 that He kept His scars not from inability to heal them, "but to wear them as an everlasting trophy of His victory." Hence Augustine says (De Civ. Dei xxii): "Perhaps in that kingdom we shall see on the bodies of the Martyrs the traces of the wounds which they bore for Christ's name: because it will not be a deformity, but a dignity in them; and a certain kind of beauty will shine in them, in the body, though not of the body." Secondly, to confirm the hearts of the disciples as to "the faith in His Resurrection" (Bede, on Luke 24:40). Thirdly, "that when He pleads for us with the Father, He may always show the manner of death He endured for us" (Bede, on Luke 24:40). Fourthly, "that He may convince those redeemed in His blood, how mercifully they have been helped, as He exposes before them the traces of the same death" (Bede, on Luke 24:40). Lastly, "that in the Judgment-day He may upbraid them with their just condemnation" (Bede, on Luke 24:40). Hence, as Augustine says (De Symb. ii): "Christ knew why He kept the scars in His body. For, as He showed them to Thomas who would not believe except he handled and saw them, so will He show His wounds to His enemies, so that He who is the Truth may convict them, saying: 'Behold the man whom you crucified; see the wounds you inflicted; recognize the side you pierced, since it was opened by you and for you, yet you would not enter.'"
Reply to Objection 1. The scars that remained in Christ's body belong neither to corruption nor defect, but to the greater increase of glory, inasmuch as they are the trophies of His power; and a special comeliness will appear in the places scarred by the wounds.
Reply to Objection 2. Although those openings of the wounds break the continuity of the tissue, still the greater beauty of glory compensates for all this, so that the body is not less entire, but more perfected. Thomas, however, not only saw, but handled the wounds, because as Pope Leo [Cf. Append. Opp. August., Serm. clxii] says: "It sufficed for his personal faith for him to have seen what he saw; but it was on our behalf that he touched what he beheld."
Reply to Objection 3. Christ willed the scars of His wounds to remain on His body, not only to confirm the faith of His disciples, but for other reasons also. From these it seems that those scars will always remain on His body; because, as Augustine says (Ad Consent., De Resurr. Carn.): "I believe our Lord's body to be in heaven, such as it was when He ascended into heaven." And Gregory (Moral. xiv) says that "if aught could be changed in Christ's body after His Resurrection, contrary to Paul's truthful teaching, then the Lord after His Resurrection returned to death; and what fool would dare to say this, save he that denies the true resurrection of the flesh?" Accordingly, it is evident that the scars which Christ showed on His body after His Resurrection, have never since been removed from His body.
Source: Summa Theologica (New Advent)