St Paul use this sort of imagery all the time, though. There is another, similar passage (Can't remember whence it came - don't keep a bible by my office desk) that starts, "And finally, my bretheren, be strong in the Lord ..." and goes on to talk about putting on "the whole armour of God."
And a favourite Methodist Minister of mine published a whole series of pamphlets entitled, The Helmet:, and The Shield: The Breastplate: which were trenchant diatribes against the iniquities of the government of the day.

I understand that the use of military metaphor is inspired, originally, in the eternal fight against Satan and all his works - that it should spill over into more mundane battles is understandable.