St Paul on the Spiritual Combat
Finally, draw your strength from the Lord and from His mighty power. Put on the armor of God so that you may be able to stand firm against the tactics of the devil. For our struggle is not with flesh and blood but with the principalities, with the powers, with the world rulers of this present darkness, with the evil spirits in the heavens.
Therefore, put on the armor of God, that you may be able to resist on the evil day and, having done everything, to hold your ground. So stand fast with your loins girded in truth, clothed with righteousness as a breastplate, and your feet shod in readiness for the gospel of peace. In all circumstances, hold faith as a shield, to quench all (the) flaming arrows of the evil one. And take the helmet of salvation and the sword of the Spirit, which is the word of God.
~ Ephesians 6:10-17