The ancient world was one of conflict, and the physical manifestation of this was often hand to hand combat. This was not an age of long-range drone warfare, but of physically exhausting, and personally dangerous battle. Armour was essential to the soldiers of that age, and as the armies of Rome patrolled the corners of the empire, their armour and equipment would be familiar features to Paul’s readers.
Ephesians chapter 6 uses this imagery. “10 Finally, be strong in the Lord and in the strength of his might. 11 Put on the whole armour of God, that you may be able to stand against the schemes of the devil. 12 For we do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places. 13 Therefore take up the whole armour of God, that you may be able to withstand in the evil day, and having done all, to stand firm. 14 Stand therefore, having fastened on the belt of truth, and having put on the breastplate of righteousness, 15 and, as shoes for your feet, having put on the readiness given by the gospel of peace. 16 In all circumstances take up the shield of faith, with which you can extinguish all the flaming darts of the evil one; 17 and take the helmet of salvation, and the sword of the Spirit, which is the word of God,”
Paul has called the saints to be ready for the struggle that this world below. This unlike the struggles of the Roman army was not a physical, but a spiritual one. The challenges of evil, and temptation more generally, are faced. These battles of faith and conscience are in many ways just as taxing as the physical. As such God has prepared us an arsenal to face it with. From head to foot we are equipped.
There is an extra factor to encourage in this battle as well. Matthew 16: 17f reads, “And Jesus answered him, “Blessed are you, Simon Bar-Jonah! For flesh and blood has not revealed this to you, but my Father who is in heaven. 18 And I tell you, you are Peter, and on this rock I will build my church, and the gates of hell shall not prevail against it.” The battle is on, and Satan (the gates of hell) shall not prevail. Gates are a defensive feature. You are on the winning side.