My rough translation of the Latin of Summa Theologiae 3.48.1. You can find another translation here. In the articles after this one, Aquinas goes on to clarify, arguing that
Christ's passion was a mean of our salvation through atonement, sacrifice, and redemption.
 We proceed to the first [objection]. It seems Christ's suffering [passio] did not cause our salvation by way of merit. For the principle of our sufferings is not in us. But no one merits or is praiseworthy save through that principle which is in them. Therefore Christ's suffering accomplished nothing by way of merit.
 Further, Christ from the beginning of His conception merited for Himself and us, as was said above (3.9.4, 3.34.3). But it is superfluous to merit again what has already been merited. Therefore Christ by His suffering did not merit our salvation.
 Further, the source of merit is charity [radix merendi est caritas]. But Christ's charity was not made greater in suffering than it was before. Therefore he did not merit our salvation by suffering more than He had already.
But contrary to this is that, on Philippians 2 (Therefore God exalted Him, etc.), Augustine says: "The humility of the passion merited glory; glory was the reward of humility." But He was glorified not only in Himself, but in His faithful, as He Himself says (John 17). Therefore it seems that He merited the salvation of His faithful [by His suffering].
I reply that it must be said that, as said above, to Christ is given grace not only as He is a singular person, but [also] insofar as He is the Head of the Churchm, so that it might redound to His members. And therefore the works of Christ are referred to Himself and His members in the same way that the works of anyone else in a state of grace are referred to him. But it is manifest that whosoever in a state of grace suffers for righteousness's sake merits his salvation thereby, according to Matthew 5: "Blessed are they who suffer persecuation for righteousness's sake." Therefore Christ by His suffering merited salvation not only for Himself, but for all His members.
To the first, therefore, it must be said, that suffering as such has an exterior principle; but inasmuch as one endures it voluntarily, it has an interior principle.
To the second, it must be said that Christ from the beginning of His conception merited our eternal salvation, but on our part there were some impediments, whereby we were prevented from receiving the effects of His preceding merits. Consequently, to remove these impediments, "it was needful for Christ to suffer," as is said above (3.46.3).
To the third, it must be said that the suffering of Christ has an effect which was not had by His preceding merits, not due to a greater charity, but due to the kind of work, which was appropriate for such an effect, as is clear from the reasons put forward for the appropriateness of Christ's passion (3.46.3).