How does Christ’s passion liberate from sin? Perhaps surprisingly, Thomas’s first answer (III, 49, 1) is entirely Abelardian: “Christ’s Passion is the proper cause of the forgiveness of sins in three ways. First of all, by way of exciting our charity . . . it is by charity that we procure pardon of our sins.”
His second answer is Augustinian, reliant on the concept of the totus Christus: Christ’s passion causes forgiveness because “He is our Head” and thus out of love “He delivered us as His members from our sins, as by the price of His Passion.” He gives the odd illustration of a man who pays off a “sin committed with his feet” by doing good work with his hands, but the point is clear. The atonement works because Jesus is the head of a body, so that whatever He does and achieves benefits the members. Ecclesiology invades soteriology.
He also claims that the passion causes forgivenes “by way of efficiency,” since the flesh of Jesus is the instrument of the Godhead, full of divine power to “expel” sins.