The phrase "Armor of God"   
is derived from Ephesians 6:11:    

 "Put on the whole armour of God, that ye may be able to stand against the wiles of the devil." (King James Version).^[1] As a biblical reference, the metaphor may refer to physical armour worn by God in metaphorical battles, or it may refer to vigilant righteousness in general as bestowed by the grace of God  "The night is far spent, the day is at hand: let us therefore cast off the works of darkness, and let us put on the armour of light."

The full quote as outlined in the King James Bible, is from Paul the Apostle's letter to the Ephesians

(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;^[1]

Given the many points of contact between the Book of Wisdom and Paul's writings (particularly his Epistle to the Romans), it is perhaps unsurprising that the imagery of the Armor of God would be used here. Compare the various elements of the whole armor of God mentioned in the epistle to the Ephesians to Wisdom 5:17–20:

17 The Lord will take his zeal as his whole armor, and will arm all creation to repel his enemies; 18 he will put on righteousness as a breast plate, and wear impartial justice as a helmet; 19 he will take holiness as an invincible shield, 20 and sharpen stern wrath for a sword, and creation will join with him to fight against his frenzied foes.

 (12) The night is nearly over; the day is almost here. So let us put aside the deeds of darkness and put on the armor of light. (13) Let us behave decently, as in the daytime, not in orgies and drunkenness, not in sexual immorality and debauchery, not in dissension and jealousy. (14) Rather, clothe yourselves with the Lord Jesus Christ, and do not think about how to gratify the desires of the sinful nature.^[2]

• belt; The Belt of Truth
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• breastplate; The Breastplate of Righteousness
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• footwear; The footwear of Peace
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• shield; The Shield of Faith
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• helmet; The Helmet of Salvation
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• sword; The Sword of the Spirit/ God's Word 

 we are told to put on the breastplate of righteousness. A breastplate shielded a warrior’s vital organs from blows that would otherwise be fatal. This righteousness is not works of righteousness done by men. Rather, this is the righteousness of Christ, imputed by God and received by faith, which guards our hearts against the accusations and charges of Satan and secures our innermost being from his attacks.

Verse 15 speaks of the preparation of the feet for spiritual conflict. In warfare, sometimes an enemy places dangerous obstacles in the path of advancing soldiers. The idea of the preparation of the gospel of peace as footwear suggests what we need to advance into Satan's territory, aware that there will be traps, with the message of grace so essential to winning souls to Christ. Satan has many obstacles placed in the path to halt the propagation of the gospel.

The shield of faith spoken of in verse 16 makes Satan's sowing of doubt about the faithfulness of God and His Word ineffective. Our faith—of which Christ is “the author and perfecter” (Hebrews 12:2)— is like a golden shield, precious, solid, and substantial.

The helmet of salvation in verse 17 is protection for the head, keeping viable a critical part of the body. We could say that our way of thinking needs preservation. The head is the seat of the mind, which, when it has laid hold of the sure gospel hope of eternal life, will not receive false doctrine or give way to Satan’s temptations. The unsaved person has no hope of warding off the blows of false doctrine because he is without the helmet of salvation and his mind is incapable of discerning between spiritual truth and spiritual deception. 

Verse 17 interprets itself as to the meaning of the sword of the Spirit—it is the Word of God. While all the other pieces of spiritual armor are defensive in nature, the sword of the Spirit is the only offensive weapon in the armor of God. It speaks of the holiness and power of the Word of God. A greater spiritual weapon is not conceivable. In Jesus' temptations in the desert, the Word of God was always His overpowering response to Satan. What a blessing that the same Word is available to us!

In verse 18, we are told to pray in the Spirit (that is, with the mind of Christ, with His heart and His priorities) in addition to wearing the full armor of God. We cannot neglect prayer, as it is the means by which we draw spiritual strength from God. Without prayer, without reliance upon God, our efforts at spiritual warfare are empty and futile. The full armor of God—truth, righteousness, the gospel, faith, salvation, the Word of God, and prayer—are the tools God has given us, through which we can be spiritually victorious, overcoming Satan’s attacks and temptations.