And your feet shod with the preparation of the gospel of peace -
Ancient warfare battles were lost or won very largely according to the weight of the masses of men that were hurled against each other; and the heavier men, with the firmer footing, were likely to be the victors. One thing that was needed was for a soldier to stand firm and resist the shock of the enemies as they rushed upon him.
In order for the soldier to stand firm (have his feet shod), he needed a good solid piece of ground to stand on, that is not slippery nor muddy. The other thing he needed was a good, strong pair of soldier’s boots, that would take hold on the ground and help him to steady himself. It also protected his feet and made it possible for him to move quickly and with certainty.
From this we can see that we're encouraged to stand firm in the midst of the battle and also allow that stance to prepare us for attack. The readiness/preparation is not necessarily zeal in proclaiming the Gospel, but being prompt in the battle.
The message of the gospel is that we're at peace with God. That peace which alone imparts a true sense of freedom, relieves us of what burdens us, and gives us courage and readiness for the battle with the enemy. We are encouraged to stand firm in the midst of the battle/challenge and face it with confidence because this gospel of peace has taught us that we have a God who fights on our behalf; a God who will never leave us nor forsake us, a God who answers prayers, a God with whom all things are possible!
We can face the battle with courage and promptitude because we know we're in covenant with God through this gospel of peace!