The ascension of Christ is the cause of our salvation in a twofold way, in one way, on our part, and in another way, on his part.
On our part, inasmuch as through the ascension of Christ our mind is moved unto him. Because through his ascension, as is said above, is given, in the first place, faith; in the second, hope; in the third, charity. Fourth, through it our reverence toward him is augmented, because we no longer value him as earthly man but as heavenly God, as the apostle says (2 Cor 5), If we have known Christ according to the flesh (i.e., as mortal), whereby we took him to be mere man (as the gloss interprets the words), but now we know him so no longer.
And on his part, with respect to that which he, ascending, did for our salvation. And first, he prepared our way for ascending into heaven, according to which it is said (Jn 14), I go to prepare you a place; and (Mic 2), He shall ascend how opens the way before them. For since he is our head, it is appropriate that the members follow where the head goes, as is said (Jn 14), and where I am, you also are. And as a sign of this, he took to heaven the souls of the saints drawn out from hell, according to the Psalm, Ascending on high, he took captivity captive, because he took those who were captured by the Devil and drew them into heaven, as into a place foreign to human nature, captured in good captivity, through his victory acquired.
Second, because, just as the priest in the Old Testament entered the sanctuary to wait on God for the people, even so Christ enters into heaven to make interecession for us, as is said (Hb 7). For the very representation of his human nature, which he brought into heaven, is a sort of intercession for us, as, from that for which God exalted human nature in Christ, he may pity those for whom the son of God assumed human nature.
Third, as, established in his heavenly seat as God and Lord, he might send divine gifts to men, as in Eph 4, He ascended above all the heavens that he might fill all, that is to say, with gifts (according to the gloss).
[Thomas Aquinas, ST 3.57.6, my rough translation. The Latin is here, the Dominican Fathers translation here. The title of this post comes from 3.57.6 ad 3, ius et dignitatem mansionis caelestis, the right and dignity of heavenly abode, which Christ's ascension has given us in perpetuity.]