St. Thomas on Justification - Christ's Resurrection is the Cause of our Justification

Our justification is not caused by anything that we do; rather, it is an effect of Christ's work on our behalf.

"In every genus that which is first is the cause of those that come after it" (Metaph. ii, 1). Now Christ, by reason of His bodily resurrection, is called "the first-fruits of them that sleep" (1 Corinthians 15:20), and "the first-begotten of the dead" (Apocalypse 1:5). Therefore His resurrection is the cause of the resurrection of others.

Further, Christ's resurrection has more in common with our bodily resurrection than with our spiritual resurrection which is by justification. But Christ's resurrection is the cause of our justification, as appears from Romans 4:25, where it is said that He "rose again for our justification." Therefore Christ's resurrection is the cause of our bodily resurrection. [ST Supp, Q76, A1; emphasis added]

(Trivia note/reminder: my namesake is the likely compiler of the Supplement to the Summa, drawing its material from Aquinas' Commentary on the Sentences of Peter Lombard)

But if Christ's resurrection is the cause of our justification, then we are not the cause of it ourselves. Therefore nothing that we do justifies us, because we do not cause it. Therefore we are not saved by our works; rather we are saved by Christ. Once again we see the error of those who falsely claim that the Gospel of Christ is works-based. They are badly misinformed.

St. Thomas further says in the same place:

Christ by reason of His nature is called the mediator of God and men: wherefore the Divine gifts are bestowed on men by means of Christ's humanity. Now just as we cannot be delivered from spiritual death save by the gift of grace bestowed by God, so neither can we be delivered from bodily death except by resurrection wrought by the Divine power. [emphasis added]