This week’s reading from Ephesians calls to mind Paul Gerhardt’s marvelous Easter hymn, “Awake, My Heart, with Gladness,” especially the fourth verse:
Now hell, it’s prince, the devil, Of all their pow’r are shorn;
Now I am safe from evil, And sin I laugh to scorn.
Grim death with all its might Cannot my soul affright;
It is a pow’rless form, Howe’er it rave and storm.
(Lutheran Service Book #467, vs. 4)
The Easter message could not be clearer: Christ Jesus has conquered sin, death, and the Devil! These three are defeated foes with no power over the risen Christ, and thus, no power over those who belong to Jesus. We are, after all, the “spoils of war” whom Christ has rescued from Satan’s slavery. As another Easter hymn boasts,
The strife is o’er, the battle done;
Now is the victor’s triumph won;
Now be the song of praise begun. Alleluia!
(The Strife Is O’er, the Battle Done, LSB #464, vs. 1)
The Pentecost season does not change the Easter proclamation but spreads that gospel abroad! Sin, death, Satan? All defeated foes! Christ is risen! Christ is Lord!
But this does not stop Satan from attacking. It does not stop him from employing the world and our flesh to convince us otherwise. Though Christ has won the victory, like a bear caught in a trap, the devil ravages and rages at anything that comes near him. Or, as Saint Peter puts it, “Your adversary the Devil prowls like a roaring lion, seeking someone to devour” (1 Peter 5:8).
What this means is the Christian life is one constantly under assault. Far from our baptism into Christ leading to the easy road of “victorious Christian living,” we experience life much more like Jesus, who immediately after being raised up from the baptismal waters was taken by the Holy Spirit to battle Satan in the wilderness (Luke 4:1-13). Baptism grants you forgiveness, life, salvation, and a very wicked foe. To be baptized is to be loved by Christ and hated by Satan. To be hated by Satan is to be under attack. To be sure, it is the attack of a defeated foe, but it is attack, nonetheless.
Baptism grants you forgiveness, life, salvation, and a very wicked foe."
Like a good field general, to conclude his epistle, Paul equips us for battle. He gives us the gifts and the armor we need to “stand against the schemes of the Devil” (Ephesians 6:11). Far from being an earthly war “against flesh and blood” (6:12), it is the “spiritual forces of evil in the heavenly places” (6:12) which assault the baptized. No soldier enters the battlefield without protection. So, Paul teaches us what it means to be clothed in the full armor of God (6:13-20), whose might is our strength (6:10).
One possible way of preaching this text is to employ the Problem/Solution structure. The problem is the fact that Satan is constantly launching flaming darts (6:16) at the saints. Christ’s victory and finding refuge in Him, being clothed in His armor in baptism, is the solution. As Professor David Schmitt warns, you must be careful in this structure not to be heavy on the problem and light on the solution.
You could break the sermon up in three problem/solution sections. The Devil is the foe who attacks. He employs the world’s temptation’s and the sinfulness continually clinging to our flesh in his assaults, all the while accusing and speaking his native lies to our consciouses. The sermon, then, would set up the three flaming darts of the Devil (the world’s temptations, the sins of our flesh, the demonic accusation of the conscience) and demonstrate how God’s armor protects us and extinguishes these arrows. In each instance, be sure to emphasize it is Christ who has won the victory and clothed us in His armor for our protection.
Christ in the Text
To see how Christ is at work in this text it is important to recognize Paul is calling the baptized to be on the defensive against Satan’s attacks, not the offensive. It is not ours to conquer Satan. Christ has accomplished this already for us on the cross. The victory is His, thus it is ours. So, when we stand firm in this armor, it is not we who have clothed ourselves to rush the battlefield in the hope we might win on the last day. The Holy Spirit puts no spears, darts, or offensive weapons in our hands with which to storm Satan’s kingdom (even the sword of the Spirit spoken of in 6:17 is referring to the Word which defends us against the lies of Satan). Rather, it is that we belong to the Victor who has clothed us in His armor to protect us while the enemy rages on. Thus, to be “strong in the Lord” is to be “in the strength of HIS might” (6:10).
It is not ours to conquer Satan. Christ has accomplished this already for us on the cross. The victory is His, thus it is ours."
There is a great deal of discussion among scholars on where Paul is drawing this armor and battle imagery from.  But, perhaps the most intriguing explanation for our purposes is how Paul’s imagery echoes Isaiah’s prophecy of YHWH fighting and winning the battle for His people:
“He saw that there was no man and wondered that there was no one to intercede; then His own arm brought Him salvation, and His righteousness upheld Him. He put on righteousness as a breastplate, and a helmet of salvation on His head; He put on garments of vengeance for clothing and wrapped Himself in zeal as a cloak” (Isaiah 59:16-17).
It is Christ, then, the incarnate God who puts on His armor and wins the battle for us. Thus victorious, He puts His armor on us to protect us from the attacks of Satan. “Paul... is alluding to the classical belief that a champion’s armor brought strength and victory to those who wore it. But he goes farther, for this is not just the armor of an ordinary human champion.”  To put on this armor is to be clothed in the battle gear of the victorious, incarnate God. “In striking Gospel irony, these enemies who are not made of blood and flesh are defeated by the flesh and blood of the divine Man Jesus Christ, crucified and risen to unite all people to Himself.” 
When preaching this text, there are two dangers to avoid. First, do not suggest Christ’s victory renders the battle obsolete. Satan really is attacking, and your parishioners deal with it every day. It can actually be a great comfort to know it is normal for the Christian to face attacks. It is not a sign of weak faith, but quite the opposite!
At the same time, do not make Satan seem so powerful that he sounds like an equal power to God. He is a defeated loser, not a divine being. The key is to recognize, though he may be stronger than us, He is not stronger than Christ. And your hearers belong to Christ, who will not let them go. What is more, the Devil’s work is all counterproductive. The more he rages at the Christian, the more the Christian flees to Christ in prayer (6:18). 
So, stand firm against the attacks of the evil one, knowing his days are numbered. Christ has crushed the serpent’s head (Genesis 3:16). The victory is ours in Jesus! Sing Easter hymns of victory!
Concordia Theology-Various helps from Concordia Seminary in St. Louis, MO to assist you in Ephesians 6:10-20.
Text Week-A treasury of resources from various traditions to help you preach Ephesians 6:10-20.