Objection 1. It would seem that Christ's Ascension is not the cause of our salvation. For, Christ was the cause of our salvation in so far as He merited it. But He merited nothing for us by His Ascension, because His Ascension belongs to the reward of His exaltation: and the same thing is not both merit and reward, just as neither are a road and its terminus the same. Therefore it seems that Christ's Ascension is not the cause of our salvation.
Objection 2. Further, if Christ's Ascension be the cause of our salvation, it seems that this is principally due to the fact that His Ascension is the cause of ours. But this was bestowed upon us by His Passion, for it is written (Hebrews 10:19): "We have [Vulgate: 'Having'] confidence in the entering into the holies by" His "blood." Therefore it seems that Christ's Ascension was not the cause of our salvation.
Objection 3. Further, the salvation which Christ bestows is an everlasting one, according to Isaiah 51:6: "My salvation shall be for ever." But Christ did not ascend into heaven to remain there eternally; for it is written (Acts 1:11): "He shall so come as you have seen Him going, into heaven." Besides, we read of Him showing Himself to many holy people on earth after He went up to heaven. to Paul, for instance (Acts 9). Consequently, it seems that Christ's Ascension is not the cause of our salvation.
On the contrary, He Himself said (John 16:7): "It is expedient to you that I go"; i.e. that I should leave you and ascend into heaven.
I answer that, Christ's Ascension is the cause of our salvation in two ways: first of all, on our part; secondly, on His.
On our part, in so far as by the Ascension our souls are uplifted to Him; because, as stated above (1, ad 3), His Ascension fosters, first, faith; secondly, hope; thirdly, charity. Fourthly, our reverence for Him is thereby increased, since we no longer deem Him an earthly man, but the God of heaven; thus the Apostle says (2 Corinthians 5:16): "If we have known Christ according to the flesh--'that is, as mortal, whereby we reputed Him as a mere man,'" as the gloss interprets the words--"but now we know Him so no longer."
On His part, in regard to those things which, in ascending, He did for our salvation. First, He prepared the way for our ascent into heaven, according to His own saying (John 14:2): "I go to prepare a place for you," and the words of Micheas (2:13), "He shall go up that shall open the way before them." For since He is our Head the members must follow whither the Head has gone: hence He said (John 14:3): "That where I am, you also may be." In sign whereof He took to heaven the souls of the saints delivered from hell, according to Psalm 67:19 (Cf. Ephesians 4:8): "Ascending on high, He led captivity captive," because He took with Him to heaven those who had been held captives by the devil--to heaven, as to a place strange to human nature. captives in deed of a happy taking, since they were acquired by His victory.
Secondly, because as the high-priest under the Old Testament entered the holy place to stand before God for the people, so also Christ entered heaven "to make intercession for us," as is said in Hebrews 7:25. Because the very showing of Himself in the human nature which He took with Him to heaven is a pleading for us. so that for the very reason that God so exalted human nature in Christ, He may take pity on them for whom the Son of God took human nature. Thirdly, that being established in His heavenly seat as God and Lord, He might send down gifts upon men, according to Ephesians 4:10: "He ascended above all the heavens, that He might fill all things," that is, "with His gifts," according to the gloss.
Reply to Objection 1. Christ's Ascension is the cause of our salvation by way not of merit, but of efficiency, as was stated above regarding His Resurrection (56, 1, ad 3,4).
Reply to Objection 2. Christ's Passion is the cause of our ascending to heaven, properly speaking, by removing the hindrance which is sin, and also by way of merit: whereas Christ's Ascension is the direct cause of our ascension, as by beginning it in Him who is our Head, with whom the members must be united.
Reply to Objection 3. Christ by once ascending into heaven acquired for Himself and for us in perpetuity the right and worthiness of a heavenly dwelling-place; which worthiness suffers in no way, if, from some special dispensation, He sometimes comes down in body to earth; either in order to show Himself to the whole world, as at the judgment; or else to show Himself particularly to some individual, e.g. in Paul's case, as we read in Acts 9. And lest any man may think that Christ was not bodily present when this occurred, the contrary is shown from what the Apostle says in 1 Corinthians 14:8, to confirm faith in the Resurrection: "Last of all He was seen also by me, as by one born out of due time": which vision would not confirm the truth of the Resurrection except he had beheld Christ's very body.
Source: Summa Theologica (New Advent)