"Then David ran and stood over the Philistine and took his sword and drew it out of its sheath and killed him and cut off his head with it. When the Philistines saw that their champion was dead, they fled." - 1 Samuel 17:51
In today's Bible verses, "the Philistines saw that their champion was dead." A champion is one who fights on behalf of the whole army. When fighting against each other, kings would often agree to spare the lives of many by having each king just send out one representative, i.e. their champion.
So instead of armies fighting and many dying, the champion from each side would meet to fight, and only one person would die. It was still army against army, but being champion against champion, it was one against one.
But when one champion defeated the other champion, the winning champion's whole team won, even though only the team champion actually fought.
Jesus is our champion. Jesus defeated the devil. We did not defeat the devil. We did not even fight the devil. Only Jesus, our champion, fought and defeated the devil. But since, through faith in Jesus, we're on Jesus's team and Jesus is our champion, we share in and therefore can claim Jesus's victory over the devil as if it were our own victory.
And through faith in Jesus, it is! Through faith in Jesus, Jesus's victory over the devil and over all God's enemies is our own!
Such is the testimony and claim for us to share in the victory of our champion, Jesus, in verse two of Martin Luther's famous 16th Century hymn, "A Mighty Fortress Is Our God." Here's the second verse:
No strength of ours can match his [the devil's] might!
We would be lost, rejected.
But now a champion comes to fight, Whom God himself elected.
You ask who this may be?
The Lord of hosts is he!
Christ Jesus, mighty Lord.
God's only Son, adored.
He holds the field victorious.
So, when the devil or any enemy of God comes to pick on you today, you can face it and literally tell it, "Jesus is my champion."
And then you can act as if the victory is already won over that enemy.
Because it is.