I answer that, Man was held captive on account of sin in two ways: first of all, by the bondage of sin, because (Jn. 8:34): "Whosoever committeth sin is the servant of sin"; and (2 Pt. 2:19): "By whom a man is overcome, of the same also he is the slave." Since, then, the devil had overcome man by inducing him to sin, man was subject to the devil's bondage. Secondly, as to the debt of punishment, to the payment of which man was held fast by God's justice: and this, too, is a kind of bondage, since it savors of bondage for a man to suffer what he does not wish, just as it is the free man's condition to apply himself to what he wills.
Since, then, Christ's Passion was a sufficient and a superabundant atonement for the sin and the debt of the human race, it was as a price at the cost of which we were freed from both obligations. For the atonement by which one satisfies for self or another is called the price, by which he ransoms himself or someone else from sin and its penalty, according to Dan. 4:24: "Redeem thou thy sins with alms." Now Christ made satisfaction, not by giving money or anything of the sort, but by bestowing what was of greatest price---Himself---for us. And therefore Christ's Passion is called our redemption.