10 Paul now begins his final instruction in this letter. His addressees are to let themselves be strengthened in Christ himself (Php 4:13). Even though victory is secure, it has to be won through battle. All the resources that the Christian soldier needs are drawn from Christ and "his mighty power." Three of the four words for power in Eph 1:19 are brought together again here. Paul's readers will recall that this is the same power that raised Jesus from the dead (1:20) and brought them to life when they were dead in trespasses and sins (2:1). Its adequacy cannot possibly be in doubt.
11 The call to "put on" God's armor recalls a similar appeal in 1 Thess 5:8. This accoutrement is provided by God and modeled on what he wears himself (Isa 11:5; 59:19). It is a complete outfit ("full armor"). The soldier must be protected from head to foot, and his armor is made up of all the various pieces, both defensive and offensive.
"Stand" is a key word in this passage (cf. vv. 13-14). It is a military term for holding on to one's position. The equipment enables the soldier to ward off the attacks of the enemy and to make a stand against him. Before any offensive can be launched, one must first of all maintain his own ground. The fourfold use of "against" stresses the determined hostility confronting the Christian soldier. The commander-in-chief of the opposing forces is the devil himself, the sworn enemy of the church. He is the master of ingenious stratagems and his tactics must not be allowed to catch us unawares. These stratagems probably reflect his deliberate attempts to destroy the unity of Christ's body (3:14-21; 4:1-16). through the invasion of false doctrine and the fomenting of dissension (4:2, 21, 31-32; 5:6).
12 In military strategy one must never underestimate the strength of the enemy. Paul is certainly not guilty of such fatal misjudgment but gives a realistic report of its potential. The "struggle" (lit., "wrestling") is not merely against human foes but a war to the death against supernatural forces.
Four aspects of the corporate menace are presented here. The particular terms used are in themselves morally neutral, though in Paul they invariably indicate something sinister (1:21; 3:10). "Rulers" are "cosmic powers." Until the end of this age these demonic forces, already defeated by Christ on the cross (4:8), exercise a certain limited authority (here "authorities") in temporarily opposing the purposes of God. The title "powers" denotes those who aspire to world control. It was attached to savior gods in the ethnic religions and identified with the sun. The expression "the spiritual forces of wickedness" suggests the heavenly bodies, which were regarded as the abode of demons who held human lives in their grip. Pagans had no option but to resign themselves to an unalterable destiny. But Christians can fight against such malign influences. "The heavenly realms" probably denotes the unseen world in general, including both good and evil forces.
13 Because the warfare in which Christians are engaged is on the scale described in v.12. the command to take advantage of the "full armor of God" is reiterated from v.11. The verb, though translated "put on," is a different one from that in v.11; here it means 'take up, assume". Thus, when the battle is at its fiercest, the soldiers of Christ will still be able to hold their line even against the most determined attack (i.e., "the day of evil"). When the emergency is over, it will be found that not an inch of territory has been yielded. Christians will "have done everything," not only in preparing for the conflict but also in pursuing it.
[NIV BIBLE COMMENTARY Volume 2: New Testament].
JESUS IS LORD.