The Phrase " Armor of God " is derived from
Ephesians 6:10-18: (10) Finally, my brethren, be strong in the Lord, and in the power of his might. (11) Put on the whole armor of God, that ye may be able to stand against the wiles of the devil. (12) For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places. (13) Wherefore take unto you the whole armor of God, that ye may be able to withstand in the evil day, and having done all, to stand. (14) Stand therefore, having your loins girt about with truth, and having on the breastplate of righteousness; (15) And your feet shod with the preparation of the gospel of peace; (16) Above all, taking the shield of faith, wherewith ye shall be able to quench all the fiery darts of the wicked. (17) And take the helmet of salvation, and the sword of the Spirit, which is the word of God: (18) Praying always with all prayer and supplication in the Spirit, and watching thereunto with all perseverance and supplication for all saints;.
In the above passage we are made to understand that our battles primarily are spiritual ones and that we need spiritual armor to be able to stand firm and come out Victoriously.
Ephesians chapter 6 mentions six pieces of armor:
belt
breastplate
footwear
shield
helmet
sword
BELT: Stand firm in the belt of Truth buckled around your waist Ephesians 6:14.
The belt of truth keeps our armor in place, the truth keeps us secure at all times.
BREASTPLATE : breastplate of salvation Ephesians 6:14. Without the breastplate of salvation the lies of the Devils penetrates our mind and wipes off our consciousness of who we are in Christ.
FOOTWEAR: ....the preparation of the gospel of Peace EPHESIANS 6:15. The footwear allows us to spread the good news of the Gospel.
SHIELD : .....the shield of faith with which you'll be able to quench all the fiery darts of the wicked one EPHESIANS 6:16. The shield as stated above enables us to quench "ALL" the fiery darts of the wicked one. 
HELMET : "take the helmet of salvation" EPHESIANS 6:17. we can ask God to place on our head the helmet of salvation so that our head is protected from the bad thoughts that the devil tries to give us.
SWORD: "....the word of God" EPHESIANS 6:17. We need to have confidence and faith in the Word of God.
PRAYER: Prayer is an often forgotten part of the armor of God, but is essential to using the armor of God well. Prayer is the way we draw strength from God and rely on Him. In essence, it is what "activates" the armor. 
People ask why do we need the armor of God?
2 Corinthians 4:8-9 explains it,
*"We are hard pressed on every side, yet not crushed; we are perplexed, but not in despair; persecuted, but not forsaken; struck down, but not destroyed"*.
God has an enemy, the devil, who does not like those who love God. To protect ourselves from the devil we are given this invisible armor. We are required to put it on every day by praying, worshiping, and by reading the Bible.
It works and we can keep it on all the time by always living for God.
As Christians, we are given a spiritual armor. You cannot see it, as it is invisible, but it works the same way as the armor that the Roman soldiers wore.
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