In part this is a reply to and confirmation of a comment made in the entry below about the Carmelite rule. I had been mulling this over for some time, and the response was the kind of confirmation I needed to go ahead and post these thoughts as disparate and tenuously connected as they are.
from The Rule of St. Albert
Since man's life on earth is a time of trial, and all who would live devotedly in Christ must undergo persecution, and the devil, your foe, is on the prowl like a roaring lion looking for prey to devour, you must use every care to clothe yourself in God's armour so that you may be ready to withstand the enemy's ambush.
The subtlety of this translation is particularly appealing. Note that the phrase used is "God's armour," not the more usual "Armor of God." This is an important difference, even thought the Latin can usually be translated either way. God's armour is the armour that belongs to God , His own battle gear, as it were. The Armor of God is armor that is not necessarily a personal possession, but rather a creation of God himself.
During our recent retreat, the retreat master went to great lengths to lay out a clear biblical exposition of the meaning and presence of God's armor in the scripture. He took great pains to make us aware that this armor was not our own armor that was "manufactured by God," but it was the very armor God himself wears when he is figuratively described in battle in a number of old-testament passages. When we clothe ourselves with it then, following the whole concept of the Simplicity of God, we are putting on God himself.
Chapter 19 of the rule goes on to give the traditional description of this armor, following closely that in Ephesians 6. What Father John-Benedict pointed out very clearly is that the vast majority of this weaponry is defensive. There is only a single offensive weapon--the sword of the word. We put on the armor to protect ourselves in the midst of the ongoing battle, not to launch an assault ourselves. The battle is the Lord's, He is the victor, and His victory is already won, we are protected by God's own armor as we walk the battlefield--but Jesus Christ wins the battle on His own merits. Our job in the battlefield is to wait and pray for all of those who have not put on the armor, who are not protected and who are not even aware that they are walking through a war zone.
Spiritual combat is never directed at another person, as Joachim notes below, it is always directed at fighting evil within us, and we do very, very little except don the armor and let God fight (see the notes on grace and will below). The spiritual battle is good vs. evil and we fight it every day in the most seemingly insignificant choices we make. Do we give alms, or do we ignore? Do we judge or do we help? Do we choose what is forbidden us, or do we accept God's commandments as a central pillar of our lives? One by one, or all at once, we face these choices in seemingly little things--for some it may be the question of whether they buy the swimsuit issue of Sports Illustrated; for others it may be returning the extra 5 dollars that came back to you by accident in change. These are small, but meaningful choices and our ability to make them in accordance with God's will is fostered by putting on His armor.
Each moment has decisions enough for a lifetime--accept God's will or reject it. And we can only perceive and understand that will when we are encased in His own armor, one body of Christ fighting the evil within ourselves by allowing the Lord to enter and win the battle, taking back the world one person at a time through His grace. So, as I concluded a day or so ago when I reopened comments--don't look to wage the battle "out there," although the battle rages there also, fight the battle within--your choices there will echo and reecho throughout the outside world, changing it slowly, subtly, bit-by-bit, to be more a reflection of what we choose moment to moment.
Deuteronomy 30:19-20:  I call heaven and earth to witness against you this day, that I have set before you life and death, blessing and curse; therefore choose life, that you and your descendants may live,
 loving the LORD your God, obeying his voice, and cleaving to him; for that means life to you and length of days, that you may dwell in the land which the LORD swore to your fathers, to Abraham, to Isaac, and to Jacob, to give them."