Objection 1. It would seem that subtlety is not a property of the glorified body. For the properties of glory surpass the properties of nature, even as the clarity of glory surpasses the clarity of the sun, which is the greatest in nature. Accordingly if subtlety be a property of the glorified body, it would seem that the glorified body will be more subtle than anything which is subtle in nature, and thus it will be "more subtle than the wind and the air," which was condemned by Gregory in the city of Constantinople, as he relates (Moral. xiv, 56).
Objection 2. Further, as heat and cold are simple qualities of bodies, i.e. of the elements, so is subtlety. But heat and other qualities of the elements will not be intensified in the glorified bodies any more than they are now, in fact, they will be more reduced to the mean. Neither, therefore, will subtlety be in them more than it is now.
Objection 3. Further, subtlety is in bodies as a result of scarcity of matter, wherefore bodies that have less matter within equal dimensions are said to be more subtle; as fire in comparison with air, and air as compared with water, and water as compared with earth. But there will be as much matter in the glorified bodies as there is now, nor will their dimensions be greater. Therefore they will not be more subtle then than now.
On the contrary, It is written (1 Corinthians 15:44): "It is sown a corruptible body, it shall rise a spiritual," i.e. a spirit-like, "body." But the subtlety of a spirit surpasses all bodily subtlety. Therefore the glorified bodies will be most subtle.
Further, the more subtle a body is the more exalted it is. But the glorified bodies will be most exalted. Therefore they will be most subtle.
I answer that, Subtlety takes its name from the power to penetrate. Hence it is said in De Gener. ii that "a subtle thing fills all the parts and the parts of parts." Now that a body has the power of penetrating may happen through two causes. First, through smallness of quantity, especially in respect of depth and breadth, but not of length, because penetration regards depth, wherefore length is not an obstacle to penetration. Secondly, through paucity of matter, wherefore rarity is synonymous with subtlety: and since in rare bodies the form is more predominant over the matter, the term "subtlety" has been transferred to those bodies which are most perfectly subject to their form, and are most fully perfected thereby: thus we speak of subtlety in the sun and moon and like bodies, just as gold and similar things may be called subtle, when they are most perfectly complete in their specific being and power. And since incorporeal things lack quantity and matter, the term "subtlety" is applied to them, not only by reason of their substance, but also on account of their power. For just as a subtle thing is said to be penetrative, for the reason that it reaches to the inmost part of a thing, so is an intellect said to be subtle because it reaches to the insight of the intrinsic principles and the hidden natural properties of a thing. In like manner a person is said to have subtle sight, because he is able to perceive by sight things of the smallest size: and the same applies to the other senses. Accordingly people have differed by ascribing subtlety to the glorified bodies in different ways.
For certain heretics, as Augustine relates (De Civ. Dei xiii, 22), ascribed to them the subtlety whereby spiritual substances are said to be subtle: and they said that at the resurrection the body will be transformed into a spirit, and that for this reason the Apostle describes as being "spiritual" the bodies of those who rise again (1 Corinthians 15:44). But this cannot be maintained. First, because a body cannot be changed into a spirit, since there is no community of matter between them: and Boethius proves this (De Duab. Nat.). Secondly, because, if this were possible, and one's body were changed into a spirit, one would not rise again a man, for a man naturally consists of a soul and body. Thirdly, because if this were the Apostle's meaning, just as he speaks of spiritual bodies, so would he speak of natural [animale] bodies, as being changed into souls [animam]: and this is clearly false.
Hence certain heretics said that the body will remain at the resurrection, but that it will be endowed with subtlety by means of rarefaction, so that human bodies in rising again will be like the air or the wind, as Gregory relates (Moral. xiv, 56). But this again cannot be maintained, because our Lord had a palpable body after the Resurrection, as appears from the last chapter of Luke, and we must believe that His body was supremely subtle. Moreover the human body will rise again with flesh and bones, as did the body of our Lord, according to Luke 24:39, "A spirit hath not flesh and bones as you see Me to have," and Job 19:26, "In my flesh I shall see God," my Saviour: and the nature of flesh and bone is incompatible with the aforesaid rarity.
Consequently another kind of subtlety must be assigned to glorified bodies, by saying that they are subtle on account of the most complete perfection of the body. But this completeness is explained by some in relation to the fifth, or heavenly, essence, which will be then predominant in them. This, however, is impossible, since first of all the fifth essence can nowise enter into the composition of a body, as we have shown above (Sent. D, 12, qu. 1). Secondly, because granted that it entered into the composition of the human body, it would be impossible to account for its having a greater predominance over the elemental nature then than now, unless--either the amount of the heavenly nature in human bodies were increased (thus human bodies would not be of the same stature, unless perhaps elemental matter in man were decreased, which is inconsistent with the integrity of those who rise again)--or unless elemental nature were endowed with the properties of the heavenly nature through the latter's dominion over the body, and in that case a natural power would be the cause of a property of glory, which seems absurd.
Hence others say that the aforesaid completeness by reason of which human bodies are said to be subtle will result from the dominion of the glorified soul (which is the form of the body) over the body, by reason of which dominion the glorified body is said to be "spiritual," as being wholly subject to the spirit. The first subjection whereby the body is subject to the soul is to the effect of its participating in its specific being, in so far as it is subject to the soul as matter to form; and secondly it is subject to the soul in respect of the other operations of the soul, in so far as the soul is a principle of movement. Consequently the first reason for spirituality in the body is subtlety, and, after that, agility and the other properties of a glorified body. Hence the Apostle, as the masters expound, in speaking of spirituality indicates subtlety: wherefore Gregory says (Moral. xiv, 56) that "the glorified body is said to be subtle as a result of a spiritual power."
This suffices for the Replies to the Objections which refer to the subtlety of rarefaction.
Source: Summa Theologica (New Advent)