Objection 1: It would seem that Christ did not ascend by His own power, because it is written (Mk. 16:19) that "the Lord Jesus, after He had spoken to them, was taken up to heaven"; and (Acts 1:9) that, "while they looked on, He was raised up, and a cloud received Him out of their sight." But what is taken up, and lifted up, appears to be moved by another. Consequently, it was not by His own power, but by another's that Christ was taken up into heaven.
Obj. 2: Further, Christ's was an earthly body, like to ours. But it is contrary to the nature of an earthly body to be borne upwards. Moreover, what is moved contrary to its nature is nowise moved by its own power. Therefore Christ did not ascend to heaven by His own power.
Obj. 3: Further, Christ's own power is Divine. But this motion does not seem to have been Divine, because, whereas the Divine power is infinite, such motion would be instantaneous; consequently, He would not have been uplifted to heaven "while" the disciples "looked on," as is stated in Acts 1:9. Therefore, it seems that Christ did not ascend to heaven by His own power.
On the contrary, It is written (Isa. 63:1): "This beautiful one in his robe, walking in the greatness of his strength." Also Gregory says in a Homily on the Ascension (xxix): "It is to be noted that we read of Elias having ascended in a chariot, that it might be shown that one who was mere man needed another's help. But we do not read of our Saviour being lifted up either in a chariot or by angels, because He who had made all things was taken up above all things by His own power."
I answer that, There is a twofold nature in Christ, to wit, the Divine and the human. Hence His own power can be accepted according to both. Likewise a twofold power can be accepted regarding His human nature: one is natural, flowing from the principles of nature; and it is quite evident that Christ did not ascend into heaven by such power as this. The other is the power of glory, which is in Christ's human nature; and it was according to this that He ascended to heaven.
Now there are some who endeavor to assign the cause of this power to the nature of the fifth essence. This, as they say, is light, which they make out to be of the composition of the human body, and by which they contend that contrary elements are reconciled; so that in the state of this mortality, elemental nature is predominant in human bodies: so that, according to the nature of this predominating element the human body is borne downwards by its own power: but in the condition of glory the heavenly nature will predominate, by whose tendency and power Christ's body and the bodies of the saints are lifted up to heaven. But we have already treated of this opinion in the First Part (Q. 76, A. 7), and shall deal with it more fully in treating of the general resurrection (Suppl., Q. 84, A. 1).
Setting this opinion aside, others assign as the cause of this power the glorified soul itself, from whose overflow the body will be glorified, as Augustine writes to Dioscorus (Ep. cxviii). For the glorified body will be so submissive to the glorified soul, that, as Augustine says (De Civ. Dei xxii), "wheresoever the spirit listeth, thither the body will be on the instant; nor will the spirit desire anything unbecoming to the soul or the body." Now it is befitting the glorified and immortal body for it to be in a heavenly place, as stated above (A. 1). Consequently, Christ's body ascended into heaven by the power of His soul willing it. But as the body is made glorious by participation with the soul, even so, as Augustine says (Tract. xxiii in Joan.), "the soul is beatified by participating in God." Consequently, the Divine power is the first source of the ascent into heaven. Therefore Christ ascended into heaven by His own power, first of all by His Divine power, and secondly by the power of His glorified soul moving His body at will.
Reply Obj. 1: As Christ is said to have risen by His own power, though He was raised to life by the power of the Father, since the Father's power is the same as the Son's; so also Christ ascended into heaven by His own power, and yet was raised up and taken up to heaven by the Father.
Reply Obj. 2: This argument proves that Christ did not ascend into heaven by His own power, i.e. that which is natural to human nature: yet He did ascend by His own power, i.e. His Divine power, as well as by His own power, i.e. the power of His beatified soul. And although to mount upwards is contrary to the nature of a human body in its present condition, in which the body is not entirely dominated by the soul, still it will not be unnatural or forced in a glorified body, whose entire nature is utterly under the control of the spirit.
Reply Obj. 3: Although the Divine power be infinite, and operate infinitely, so far as the worker is concerned, still the effect thereof is received in things according to their capacity, and as God disposes. Now a body is incapable of being moved locally in an instant, because it must be commensurate with space, according to the division of which time is reckoned, as is proved in Physics vi. Consequently, it is not necessary for a body moved by God to be moved instantaneously, but with such speed as God disposes.