The apostle shows how righteousness acts to be protective of the heart or affections in a saved person – a large target of the devil. Here is how self-denial, love for Christ and obedience to the commandments garrison the heart from pollution and shame.
The foundational piece of the Christian’s armour is firstly objective truth – the unchanging doctrines of the faith. (The sword, later, is Scripture used ‘offensively’ to make Christ known). Secondly, the belt is also subjective truth, truthfulness and sincerity for which here are helpful rules.