Christ overcame the world by casting out the ruler of the world: "Now shall the ruler of this world be cast out" (John 12:31); "He disarmed the principalities and powers" (Colossians 2:15). This shows us that the devil is also to be overcome by us: "Will you play with him as with a bird, or will you put him on a leash for your maidens?" (Job 41:5), which understood literally means that after the passion of Christ, the little boys and young handmaids of Christ will make him [the devil] their plaything. (Super evangelium S. Joannis lectura, section 2176; trans. Fabian Larcher)
We're probably all familiar with the idea that Jesus has in some sense won a huge victory. After all, like Paul said in one of the passages Thomas Aquinas already cited, "Having disarmed the powers and authorities, he made a public spectacle of them, triumphing over them by the cross" (Colossians 2:15). Thomas' specific comments on that passage in his commentary on Colossians are pretty verbose and most consist of quoting other Bible passages, but the basic idea is that Christ whupped the demonic powers somethin' fierce on Calvary that day and freed us and our forebears in the faith from the bondage of Satan, death, and sin. There's a strong tradition of viewing the atonement at least partially as Christus Victor, Christ the Victor, so that the atonement is Christ decisively beating the forces of evil by his death on the cross. (I'm not saying, like Gustaf Aulen did in his 'classic' study of the idea, that this way of seeing things exhausts the atonement; there are certainly elements of penal substitution or satisfaction involved, or something along those lines. Just to make sure I'm clear, you know.)
And we often think of ourselves as in some sense victorious. After all, Paul says, "Thanks be to God! He gives us the victory through our Lord Jesus Christ" (1 Corinthians 15:57), and who hasn't heard Eugene Bartlett's 20th-century hymn "Victory in Jesus"?
My Savior, forever.
He sought me and bought me
With his redeeming blood;
He loved me 'ere I knew him,
And all my love is due him;
He plunged me to victory
Beneath the cleansing flood.
But how often do we really think about the idea of sharing Christ's victory in any concrete way? I mean, yeah, we often think of it in abstract ways, like freedom from death--though even that's kinda tough to wrap your head around. That's what I love about the passage I quoted from Thomas Aquinas. I've never before thought of the idea of Christ's victory--and ours with him--as being so complete as to basically declaw, de-fang, and domesticate even the demons. Mostly, I just really love the triumphant imagery of Satan being so thoroughly defeated in the end that even little kids could basically put him on a leash and take him for a walk. In today's Christian circles, we're sometimes so worried about staving off Satan's attacks, real or perceived, that we come to fear him. And we should be wary, we should be vigilant. Don't mistake my words as an excuse to lose focus. Satan ain't no tame beast--yet. (Even though Thomas Aquinas' statement could be understood that way, I must admit.) But when I think of Satan's power being that restricted in the age to come, when I think of our victory being that total, I can't help but laugh for joy and shout praise to the Lord who fought the battle that won the victory and to the Father who passed it on to us, the faithful.
Don't let your guard down. Don't think that the forces of evil are obliterated and that we're already in paradise. There are still plenty of dangers out there for us. But don't make the opposite mistake of thinking that "Satan is alive and well on planet earth", to borrow the title of a horrible end-times book; he may be kicking, but after the death and resurrection of Jesus Christ, Satan is definitely not doing "well". And while there remain threats out there, if we're faithful, we have nothing to fear.