A lot of commentators analyze the actual armour, like the belt, shield or sword Paul is referring to, this, in my opinion, is a colossal waste of space and misses the entire purpose. What I want to do is talk about what Paul means by truth, righteousness and salvation. And these ideas do not come out of the Reformation (i.e. imputed righteousness) or the Greek/modern definition of truth, these ideas are found in Isaiah, and that is where we should look.
I will deal with "Truth" in detail and then only briefly highlight righteousness and salvation.
This is the tricky part because the Greek word which translated the Hebrew word looses a lot of it's meaning by the time it reaches the English translation. We don't think about it often but, in English, someone or something can be true in more than one sense. For instance: an accusation can be true, or I can be true to my wife. They both mean very different things, but use the same word. This ambiguity can also be present in the Greek word, however it usually means something like the former. How do we find out what Paul meant?
If the Old Testament is ignored the most common meaning would naturally be adopted, and since that meaning is the same in Greek and English "truth" comes to mean something like "believing the truth". In fact, this idea is so prevalent that one of the major Greek lexicons for the New Testament and early writings (known as BDAG) defines truth in this passage as the "truth of the Christian gospel". This is wrong.
If you remember Isaiah 11:5 (I quoted it in the first blog), you will realize that the English translation doesn't translate the word "truth" as such but as "faithfulness". This is because the Hebrew is "true" in the relational sense, and never means "truth" in the Greek/modern sense. Paul uses a Greek translation here and the meaning gets lost, but looking back to where Paul is citing from it's clear he meant "faithfulness" not "truth". Therefore it is faithfulness to God and Christ that serves as a belt.
Righteousness is easier to handle and a direct reading of the English texts should make the meaning obvious. Traditionally "righteousness" has either been thought of as the imputed righteousness of Christ or righteousness as a virtue (i.e. private holiness). But in understanding the context of Isaiah 59 one realizes that both are mistaken. In Isaiah, God is interested in helping bring justice to Israel because no one else is doing it. The Christian, in Ephesians, is charged with helping God bring justice and righteousness to those around them.
Salvation is a tough one. To be honest, there are probably 2 equally valid readings (although they are not exclusive) based on Isaiah. The first is that Christians bring salvation to those who will accept it, the second is that Christians can be assured of their salvation. Both, I think, can be read as from the Isaiah passage.
The last blog will quickly put all this into context of Ephesians 6, however, I think where this leads is quite evident already.