Take up the full armor of God... the belt of truth, the breastplate of righteousness, the shoes of preparation of the gospel of peace, the shield of faith, the helmet of salvation, the sword of the Spirit.
The context of this passage is that our battle is not against flesh and blood, but against worldly darkness and the spiritual forces of evil. Our job is to resist the dark evil, to stand firm, clothed in full spiritual armor, praying at all times in the Spirit.
But since we're not very well versed in the Jewish Scriptures, we fail to realize that Paul probably had in the back of his mind, and intended to remind his readers of, a passage in Isaiah 59:17, in which context God looks around and sees the evil in the world, and taking it upon himself to put on the breastplate of righteousness and the helmet of salvation and the robe of vengeance, then goes out to repay fury to his adversaries and recompense to his enemies according to their deeds.
In Isaiah, God is wearing the armor, and takes physical vengeance on evil-doers. In Ephesians, we, on God's side, wear our own set of armor, but not the robe of vengeance, and our battle is not physical but spiritual.
I think Paul's readers would have connected these two passages; I think maybe we should also.