Objection 1. It would seem that we were not delivered from the power of the devil through Christ's Passion. For he has no power over others, who can do nothing to them without the sanction of another. But without the Divine permission the devil could never do hurt to any man, as is evident in the instance of Job (1,2), where, by power received from God, the devil first injured him in his possessions, and afterwards in his body. In like manner it is stated (Matthew 8:31-32) that the devils could not enter into the swine except with Christ's leave. Therefore the devil never had power over men: and hence we are not delivered from his power through Christ's Passion.
Objection 2. Further, the devil exercises his power over men by tempting them and molesting their bodies. But even after the Passion he continues to do the same to men. Therefore we are not delivered from his power through Christ's Passion.
Objection 3. Further, the might of Christ's Passion endures for ever, as, according to Hebrews 10:14: "By one oblation He hath perfected for ever them that are sanctified." But deliverance rom the devil's power is not found everywhere, since there are still idolaters in many regions of the world; nor will it endure for ever, because in the time of Antichrist he will be especially active in using his power to the hurt of men; because it is said of him (2 Thessalonians 2:9): "Whose coming is according to the working of Satan, in all power, and signs, and lying wonders, and in all seduction of iniquity." Consequently it seems that Christ's Passion is not the cause of the human race being delivered from the power of the devil.
On the contrary, our Lord said (John 12:31), when His Passion was drawing nigh: "Now shall the prince of this world be cast out; and I, if I be lifted up from the earth, will draw all things to Myself." Now He was lifted up from the earth by His Passion on the cross. Therefore by His Passion the devil was deprived of his power over man.
I answer that, There are three things to be considered regarding the power which the devil exercised over men previous to Christ's Passion. The first is on man's own part, who by his sin deserved to be delivered over to the devil's power, and was overcome by his tempting. Another point is on God's part, whom man had offended by sinning, and who with justice left man under the devil's power. The third is on the devil's part, who out of his most wicked will hindered man from securing his salvation.
As to the first point, by Christ's Passion man was delivered from the devil's power, in so far as the Passion is the cause of the forgiveness of sins, as stated above (Article 1). As to the second, it must be said that Christ's Passion freed us from the devil's power, inasmuch as it reconciled us with God, as shall be shown later (4). But as to the third, Christ's Passion delivered us from the devil, inasmuch as in Christ's Passion he exceeded the limit of power assigned him by God, by conspiring to bring about Christ's death, Who, being sinless, did not deserve to die. Hence Augustine says (De Trin. xiii, cap. xiv): "The devil was vanquished by Christ's justice: because, while discovering in Him nothing deserving of death, nevertheless he slew Him. And it is certainly just that the debtors whom he held captive should be set at liberty since they believed in Him whom the devil slew, though He was no debtor."
Reply to Objection 1. The devil is said to have had such power over men not as though he were able to injure them without God's sanction, but because he was justly permitted to injure men whom by tempting he had induced to give consent.
Reply to Objection 2. God so permitting it, the devil can still tempt men's souls and harass their bodies: yet there is a remedy provided for man through Christ's Passion, whereby he can safeguard himself against the enemy's assaults, so as not to be dragged down into the destruction of everlasting death. And all who resisted the devil previous to the Passion were enabled to do so through faith in the Passion, although it was not yet accomplished. Yet in one respect no one was able to escape the devil's hands, i.e. so as not to descend into hell. But after Christ's Passion, men can defend themselves from this by its power.
Reply to Objection 3. God permits the devil to deceive men by certain persons, and in times and places, according to the hidden motive of His judgments; still, there is always a remedy provided through Christ's Passion, for defending themselves against the wicked snares of the demons, even in Antichrist's time. But if any man neglect to make use of this remedy, it detracts nothing from the efficacy of Christ's Passion.
Source: Summa Theologica (New Advent)