Objection 1. It would seem that it was not fitting for Christ to ascend into heaven. For the Philosopher says (De Coelo ii) that "things which are in a state of perfection possess their good without movement." But Christ was in a state of perfection, since He is the Sovereign Good in respect of His Divine Nature, and sovereignly glorified in respect of His human nature. Consequently, He has His good without movement. But ascension is movement. Therefore it was not fitting for Christ to ascend.
Objection 2. Further, whatever is moved, is moved on account of something better. But it was no better thing for Christ to be in heaven than upon earth, because He gained nothing either in soul or in body by being in heaven. Therefore it seems that Christ should not have ascended into heaven.
Objection 3. Further, the Son of God took human flesh for our salvation. But it would have been more beneficial for men if He had tarried always with us upon earth; thus He said to His disciples (Luke 17:22): "The days will come when you shall desire to see one day of the Son of man; and you shall not see it." Therefore it seems unfitting for Christ to have ascended into heaven.
Objection 4. Further, as Gregory says (Moral. xiv), Christ's body was in no way changed after the Resurrection. But He did not ascend into heaven immediately after rising again, for He said after the Resurrection (John 20:17): "I am not yet ascended to My Father." Therefore it seems that neither should He have ascended after forty days.
On the contrary, Are the words of our Lord (John 20:17): "I ascend to My Father and to your Father."
I answer that, The place ought to be in keeping with what is contained therein. Now by His Resurrection Christ entered upon an immortal and incorruptible life. But whereas our dwelling-place is one of generation and corruption, the heavenly place is one of incorruption. And consequently it was not fitting that Christ should remain upon earth after the Resurrection; but it was fitting that He should ascend to heaven.
Reply to Objection 1. That which is best and possesses its good without movement is God Himself, because He is utterly unchangeable, according to Malachi 3:6: "I am the Lord, and I change not." But every creature is changeable in some respect, as is evident from Augustine (Gen. ad lit. viii). And since the nature assumed by the Son of God remained a creature, as is clear from what was said above (2, 7; 16, 8,10; 20, 1), it is not unbecoming if some movement be attributed to it.
Reply to Objection 2. By ascending into heaven Christ acquired no addition to His essential glory either in body or in soul: nevertheless He did acquire something as to the fittingness of place, which pertains to the well-being of glory: not that His body acquired anything from a heavenly body by way of perfection or preservation; but merely out of a certain fittingness. Now this in a measure belonged to His glory; and He had a certain kind of joy from such fittingness, not indeed that He then began to derive joy from it when He ascended into heaven, but that He rejoiced thereat in a new way, as at a thing completed. Hence, on Psalm 15:11: "At Thy right hand are delights even unto the end," the gloss says: "I shall delight in sitting nigh to Thee, when I shall be taken away from the sight of men."
Reply to Objection 3. Although Christ's bodily presence was withdrawn from the faithful by the Ascension, still the presence of His Godhead is ever with the faithful, as He Himself says (Matthew 28:20): "Behold, I am with you all days, even to the consummation of the world." For, "by ascending into heaven He did not abandon those whom He adopted," as Pope Leo says (De Resurrec., Serm. ii). But Christ's Ascension into heaven, whereby He withdrew His bodily presence from us, was more profitable for us than His bodily presence would have been.
First of all, in order to increase our faith, which is of things unseen. Hence our Lord said (John 16) that the Holy Ghost shall come and "convince the world... of justice," that is, of the justice "of those that believe," as Augustine says (Tract. xcv super Joan.): "For even to put the faithful beside the unbeliever is to put the unbeliever to shame"; wherefore he goes on to say (10): "'Because I go to the Father; and you shall see Me no longer'"--"For 'blessed are they that see not, yet believe.' Hence it is of our justice that the world is reproved: because 'you will believe in Me whom you shall not see.'"
Secondly, to uplift our hope: hence He says (John 14:3): "If I shall go, and prepare a place for you, I will come again, and will take you to Myself; that where I am, you also may be." For by placing in heaven the human nature which He assumed, Christ gave us the hope of going thither; since "wheresoever the body shall be, there shall the eagles also be gathered together," as is written in Matthew 24:28. Hence it is written likewise (Micah 2:13): "He shall go up that shall open the way before them."
Thirdly, in order to direct the fervor of our charity to heavenly things. Hence the Apostle says (Colossians 3:1-2): "Seek the things that are above, where Christ is sitting at the right hand of God. Mind the things that are above, not the things that are upon the earth": for as is said (Matthew 6:21): "Where thy treasure is, there is thy heart also." And since the Holy Ghost is love drawing us up to heavenly things, therefore our Lord said to His disciples (John 16:7): "It is expedient to you that I go; for if I go not, the Paraclete will not come to you; but if I go, I will send Him to you." On which words Augustine says (Tract. xciv super Joan.): "Ye cannot receive the Spirit, so long as ye persist in knowing Christ according to the flesh. But when Christ withdrew in body, not only the Holy Ghost, but both Father and Son were present with them spiritually."
Reply to Objection 4. Although a heavenly place befitted Christ when He rose to immortal life, nevertheless He delayed the Ascension in order to confirm the truth of His Resurrection. Hence it is written (Acts 1:3), that "He showed Himself alive after His Passion, by many proofs, for forty days appearing to them": upon which the gloss says that "because He was dead for forty hours, during forty days He established the fact of His being alive again. Or the forty days may be understood as a figure of this world, wherein Christ dwells in His Church: inasmuch as man is made out of the four elements, and is cautioned not to transgress the Decalogue."
Source: Summa Theologica (New Advent)