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.
Thomas Aquinas, ST III.57.6c