He has already taught at length how to overcome the passions, first anger and then desire. He also taught what Christians ought to be like amid the world. Now he arms them against the chief enemies. They usually enter the struggle in their own person after the flesh and the world prove powerless against a zealous Christian.
"Finally." Now I have said everything. What remains is to inspire you for battle and arm you. "My brethren." You are not alone. I too am with you. It is as though he said: come, brothers, let us stand firm.
"Be strong," ενδυναμουσθε, can mean "be filled with strength," "put on strength," and "gather your strength." Be inspired with courage, dare, and go out to battle without timidity. This is the same as the Apostle says elsewhere: "Be of good courage, and let your heart be strengthened" (Ps. 30:25). Or as the Lord Himself inspired the Apostles: "Be of good cheer, for I have overcome the world" (Jn. 16:33).
Trusting in Him, the Apostle also teaches them to be strong, saying, "be strong." "In the Lord, and in the might of His strength." This is where strengthening lies. It is in the Lord and in the strong might of His power, in His almighty authority by which He sustains all and governs all.
Such is the Lord, and He is our Lord, and we are His. He will not allow His own to be wronged. The Lord is our helper. We will not fear. What can they do to us? If God is for us, who is against us? Let us be bold and courageous. With such a Leader and Protector, what hostile power can overcome us?
"Be strong in the Lord," that is, in hope in Him, through His help. He speaks as though saying: "Do not fear. Place your hope in God, and He will make everything easy for you" (St. Chrysostom). "Put away every fear, guarded by the power of God" (Theodoret).
When the Apostle says, "be strong in the Lord," he does not command them to seek help, but to take courage while trusting in ready help. The Lord is already with you and is ready to defend you and scatter the enemies. Only do not be timid.
Timidity will mean a weakening of hope in the Lord, an inner betrayal of Him. It is as though His power were diminished before the face of the enemies. After this diminishing comes a diminishing of help and protection, and therefore stronger assaults from the enemies. But for one who wholly trusts in the Lord and is devoted to Him, one gesture from the Lord scatters all their armies.