Objection 1. It would seem that the Son of God did not assume a true body. For it is written (Philippians 2:7), that He was "made in the likeness of men." But what is something in truth is not said to be in the likeness thereof. Therefore the Son of God did not assume a true body.
Objection 2. Further, the assumption of a body in no way diminishes the dignity of the Godhead; for Pope Leo says (Serm. de Nativ.) that "the glorification did not absorb the lesser nature, nor did the assumption lessen the higher." But it pertains to the dignity of God to be altogether separated from bodies. Therefore it seems that by the assumption God was not united to a body.
Objection 3. Further, signs ought to correspond to the realities. But the apparitions of the Old Testament which were signs of the manifestation of Christ were not in a real body, but by visions in the imagination, as is plain from Isaiah 6:1: "I saw the Lord sitting," etc. Hence it would seem that the apparition of the Son of God in the world was not in a real body, but only in imagination.
On the contrary, Augustine says (Qq. lxxxiii, qu. 13): "If the body of Christ was a phantom, Christ deceived us, and if He deceived us, He is not the Truth. But Christ is the Truth. Therefore His body was not a phantom." Hence it is plain that He assumed a true body.
I answer that, As is said (De Eccles. Dogm. ii). The Son of God was not born in appearance only, as if He had an imaginary body; but His body was real. The proof of this is threefold. First, from the essence of human nature to which it pertains to have a true body. Therefore granted, as already proved (4, 1), that it was fitting for the Son of God to assume human nature, He must consequently have assumed a real body. The second reason is taken from what was done in the mystery of the Incarnation. For if His body was not real but imaginary, He neither underwent a real death, nor of those things which the Evangelists recount of Him, did He do any in very truth, but only in appearance; and hence it would also follow that the real salvation of man has not taken place; since the effect must be proportionate to the cause. The third reason is taken from the dignity of the Person assuming, Whom it did not become to have anything fictitious in His work, since He is the Truth. Hence our Lord Himself deigned to refute this error (Luke 24:37-39), when the disciples, "troubled and frighted, supposed that they saw a spirit," and not a true body; wherefore He offered Himself to their touch, saying: "Handle, and see; for a spirit hath not flesh and bones, as you see Me to have."
Reply to Objection 1. This likeness indicates the truth of the human nature in Christ--just as all that truly exist in human nature are said to be like in species--and not a mere imaginary likeness. In proof of this the Apostle subjoins (Philippians 2:8) that He became "obedient unto death, even to the death of the cross"; which would have been impossible, had it been only an imaginary likeness.
Reply to Objection 2. By assuming a true body the dignity of the Son of God is nowise lessened. Hence Augustine [Fulgentius] says (De Fide ad Petrum ii): "He emptied Himself, taking the form of a servant, that He might become a servant; yet did He not lose the fulness of the form of God." For the Son of God assumed a true body, not so as to become the form of a body, which is repugnant to the Divine simplicity and purity--for this would be to assume a body to the unity of the nature, which is impossible, as is plain from what has been stated above (Question 2, Article 1): but, the natures remaining distinct, He assumed a body to the unity of Person.
Reply to Objection 3. The figure ought to correspond to the reality as regards the likeness and not as regards the truth of the thing. For if they were alike in all points, it would no longer be a likeness but the reality itself, as Damascene says (De Fide Orth. iii, 26). Hence it was more fitting that the apparitions of the old Testament should be in appearance only, being figures; and that the apparition of the Son of God in the world should be in a real body, being the thing prefigured by these figures. Hence the Apostle says (Colossians 2:17): "Which are a shadow of things to come, but the body is Christ's."
Source: Summa Theologica (New Advent)