Ephesians 6:10

Here is a general exhortation to constancy in our Christian course, and to encourage in our Christian warfare. Is not our life a warfare? It is so; for we struggle with the common calamities of human life. Is not our religion much more a warfare? It is so; for we struggle with the opposition of the powers of darkness, and with many enemies who would keep us from God and heaven. We have enemies to fight against, a captain to fight for, a banner to fight under, and certain rules of war by which we are to govern ourselves. "Finally, my brethren (Eph 6:10), it yet remains that you apply yourselves to your work and duty as Christian soldiers." Now it is requisite that a soldier be both stout-hearted and well armed. If Christians be soldiers of Jesus Christ, I. They must see that they be stout-hearted. This is prescribed here: Be strong in the Lord, etc. Those who have so many battles to fight, and who, in their way to heaven, must dispute every pass, with dint of sword, have need of a great deal of courage. Be strong therefore, strong for service, strong for suffering, strong for fighting. Let a soldier be ever so well armed without, if he have not within a good heart, his armour will stand him in little stead. Note, spiritual strength and courage are very necessary for our spiritual warfare.

— Matthew Henry's Concise Commentary (public domain)

my brethren--Some of the oldest manuscripts omit these words. Some with Vulgate retain them. The phrase occurs nowhere else in the Epistle (see, however, Eph 6:23); if genuine, it is appropriate here in the close of the Epistle, where he is urging his fellow soldiers to the good fight in the Christian armor. Most of the oldest manuscripts for "finally," read, "henceforward," or "from henceforth" (Gal 6:17). be strong--Greek, "be strengthened." in the power of his might--Christ's might: as in Eph 1:19, it is the Father's might.

— Jamieson-Fausset-Brown Commentary (public domain)