Ephesians like Romans and Galatians splits neatly into two parts, with the second giving the practical outworkings of the first. Here opening (chs. 1-4) is “indicative” – it tells us what is – the second (chs. 5-6) is “imperative” – it tells us what to do.
The letter has strong similarities to Colossians, some think it is an expansion, filling out the earlier letter. It is written with some long complicated sentences, and uses words and phrases that Paul does not use elsewhere. So some scholars think it is not by Paul. (My Greek is not up to deciding this, so I’ll call the author “Paul” since that is what he calls himself 😉
Generations of children’s talks have conditioned individualistic Westerners to hear this “armour of God” section as all about individual piety. But the collective nature of ancient cultures, the stress on unity and community in the letter, the echo of “put on Christ” in “put on the whole armour” suggests we read it more collectively.
There is debate over whether this Peroratio (the summary section just before the ending of ancient letters) only sums up the “imperative” section, or whether it sums up the whole. ((Great names like Aquinas, Calvin and FF Bruce supported the “only the Imperative section” view, while already Jerome saw it summing up the whole letter in the usual way.)) Looking for the key words (e.g. for the “breastplate of righteousness” look for “righteous*”) earlier in the letter seems to me to make it clear it sums up (Notice too the place of this concept in the glorious opening of the letter – which you surely MUST read as well as these verses!) the whole.
When we put on Christ (in putting on all these “bits” of armour) we change our relationship with the world, and in doing so we resist the power of the rulers of this world!