If I were a soldier in a nation’s army, I would have been taught the basic tenants of how to act as a soldier in boot camp and other early training.
When an event arose that thrust me into a situation of my nation being at war, I would need those basic teachings to be well memorized, so that I moved in unity with the rest of the army as we functioned together in the war.
The even more valuable second layer of instruction would be the orders that proceeded from headquarters in the working out of their plan for victory. No matter how great my preparation was, if I was too fearful or rebellious to obey the fresh orders, I would be useless at best, and detrimental at worst.
Paul had warfare in mind when he wrote a letter to the churches at Ephesus. In their city, he had encountered some of his most effective and most dangerous wartime experiences. The advice he gave them regarding their continuing warfare in that area included what we call chapter 6, and a list of instructions we call putting on “the full armor of God.”
Most teaching on this Scripture gives great understanding of the armor pieces that he mentioned, and how spiritual application of the ideas could help us in our stands against the wiles of the devil. I can’t remember but two out of the scores of commentaries I have encountered that include verse 18 with 10-17 as part of the narrative. There, he wrote, “praying always with all prayer and supplication in the Spirit,…”
His uses of the phrase, “In the Spirit” in other Scriptures are unquestioned as meaning under the guidance of the Spirit of God. Walking in the Spirit, beginning in the Spirit, being in the Spirit (Galatians 3:3, 5:16, 5:25, Romans 8:9). Here, we should have the same idea. “Praying always under the guidance of the Holy Spirit.”
A well prepared, well dressed, well armored soldier is a museum piece without a commander giving her/him orders that make them useful in the conflict, co-operating with the rest of the soldiers, unified against an enemy and for victory. Even when I do manage to listen to the Spirit well enough to sort out the instructions He has for me in the Scriptures, if I am not listening to Him for daily directions, I will be useless or detrimental in His plans in the earth.