Put on the breastplate of righteousness and helmet of salvation - (I combined the two because it's coupled in 1 Thessalonians 5:8 and Isaiah 59:17)
Both the breastplate and the helmet were used as protective equipment, guarding against any blows to the chest or head area. Both areas are vitally important to the human body and any severe blow to either could lead to death. This is significant because as Christians, two very important areas in our spiritual lives are the heart and mind.
These two subjects are very closely related.
The bible is filled with verses about these subjects and how important they are if we want to develop spiritually (above all else, guard your heart, renewing our minds, the parable of the soil); so we recognise the priority that we should give to those areas.
With regards to the armour however, one could say that our hearts are protected/guarded by righteousness and our minds are protected/guarded by salvation.
Why is that?
One hindrance to developing spiritually, especially concerning knowing that you are loved by God, knowing what He's made available to us as His children and who we are in Christ, is having a belief and mindset that says "I'm not worthy; I'll never amount to anything; how could anyone ever love me", etc.
It's the complete opposite of what we're taught in the word of God concerning what righteousness and salvation means to us. For it teaches us that we're forgiven, accepted, blessed, cared for, in covenant with God and loved by our Father; no longer having to feel insecure, depressed, helpless and alone, all because of what Christ did for us by paying the penalty for our sins.
We are to guard ourselves against these thoughts by knowing what Righteousness and Salvation means to us. God loves us - this is the starting point of it all. It's the starting point of faith and believing in the promises of God!
God loves you! Let that fill your heart and mind!