Paul reminds us that only in the Authority and power of Jesus is their victory in this spiritual war. He then tells us who our enemy really is: they are the demons behind the evil people who use these people for their own purposes.
He also advises us that God has given us protection in this battle with Satan and that only as we take on the whole armour (protection) of God will we succeed in this spiritual battle.
Or as the Syriac Peshita says:
We are to know the truths of God which means we will understand what the battle is all about and how to fight I and the lies of the enemy and need to be in a correct relationship with God to successfully fight this battle.
The breastplate protects the heart and our righteous (correct) relationship with God protects our spiritual heart as Satan cannot attack us successfully if we are in a righteous relationship with God.
The Gospel attacks Satanís kingdom freeing people from it so is a form of spiritual warfare and the main purpose for the fighting we do with Satan. The purpose of God is to have everyone at peace with Him which is why we need to know The Gospel and be able to use (explain) its truths to defeat the enemy and his purpose for people.
Faith in Jesus places us in a position for Him to protect us and to help us fight the battle. So what faith is it in?
Otherwise you will not fight the battle or may not even know there is one. Remember it is the truth that sets you free from the lies of Satan.
The helmet covers the brain (mind) and if we are saved we are in a correct relationship with Jesus and understand this relationship correctly so that Satan cannot remove us from it through his lies.
The sword of The Spirit is the Word of God (The Bible) which tells us the truths we need to know to take part in this battle. If you do not know your Bible you will not be able to battle in the way you should because you will not have the knowledge of the truth to do this
Always communicating to God your needs and asking His advice in which we are helped by The Holy Spirit (Rom 8:26-27) If we are not able to communicate with our commander and hear Him how are we to know what He desires us to do which is why we need always to be guided by The Holy Spirit and be always alert to hear His instructions.
We need to pray for each other and help each other in this battle. Satan is attacking us and we are The Body of Christ so the Body needs to unite as one in purpose to defeat Satan. This means we need to help each other in this battle either to protect each other from Satanís attacks or fight for and with them against Satan.
Paul mentions The Gospel at the end as that is our main weapon to rescue people from the kingdom of Satan. Without the truths of The Gospel they have no reason to go from Satanís kingdom to The Kingdom of God.
Many Christians have no idea about this battle which is why Jesus said only 10% of Christians would go up if the rapture occurred at this at the moment. They know about Him, He said, but do not have a personal relationship with Him which means they are not in a position to be able to even fight the battle. After the three greast revivals Jesus said only 20% would go up.
Unless you are fighting the battle or are constantly on the alert for Satanís attacks then you are a victim of it and cannot be really used by God.
It is also no use fighting the battle if you have nothing better to offer them. This is why the truths of The Gospel are central to this battle.