These verses form a powerful and eloquent conclusion to the letter.
They represent a reworking of biblical imagery to support the author's viewpoint that the church, having been united as one body through the work of Christ, takes an active role in pursuing God's righteousness and justice.
The author is working closely with Isaiah 59, in which God appears as a divine warrior who will bring about justice (see Isaiah 59:15b-19; this tradition is also developed in Wisdom of Solomon 5:17-23 and 1 Thessalonians 5:8). As in Isaiah 59:17, the "armor of God" includes "the breastplate of righteousness" (Ephesians 6:14), and "the helmet of salvation" (Ephesians 6:17). Yet in Ephesians' use of the imagery, it is the community of the faithful that takes up this armor. They do so "in the Lord," which suggests that they wage this battle alongside the Lord. However, in Isaiah the language indicates that "there was no one to intervene" (59:16), and so God fights for justice alone. In Ephesians, we see a unique expression of the church's role in salvation and justice. By taking up God's armor, the community becomes active in the struggle against spiritual forces (cf. verse 12).
The active role of the community is also communicated in the verbs of verses 10-11. "Be strong" (verse 10; Greek: endunamousthe) and "clothe yourselves" (verse 11; Greek: endusasthe) are both middle verbs in Greek, which gives them a reflexive quality: literally, "strengthen yourselves" as well as "clothe yourselves." In the author's earlier discussions of God's power, the active role of the community has not been stated explicitly. This power was put to work in Christ (1:19-20) and is far above other powers (1:21). The author prays that the readers may be strengthened with God's power (3:16) but also recognizes the "power at work within us" (3:20). Ephesians 6:10-11 extends this notion to suggest that the community itself acts to take up God's power, at least partially through its own initiative.
The active role of the church is not altogether surprising, given the writer's previous indication that God has "raised us up with [Christ] and seated us with him in the heavenly places" (2:6). This exaltation is a unique expression of the church's identity among New Testament writings. However, it is interesting to note that, while Christians are already seated with Christ in the heavenly places, this position does not eliminate the need for struggle. The wrestling "against the spiritual forces of evil" also takes place "in the heavenly places" (6:12).
While modern Christians are likely to have a view of heaven as a paradise in which no evil dwells, the writer of Ephesians is drawing on a different set of cultural assumptions, one in which a struggle between cosmic forces occurs within the heavenly realm. Christians, who already reign with Christ in some sense, are obligated to participate in this struggle.
The armor of God that the church takes up relates to the message that the author has already laid out. The theological message of Ephesians 1-3 is now depicted metaphorically as preparation for a spiritual battle in which believers engage through their actions. By girding themselves with the "belt of truth" (6:14), readers metaphorically prepare themselves for the work to which they have already been called: they are to "speak the truth in love" to one another (cf. 4:15, 25). Similarly the "breastplate of righteousness" relates to the "new self" with which they are to clothe themselves, as beings "created according to the likeness of God in true righteousness and holiness" (4:24).
The author has earlier explained the "gospel of peace" (6:15), for which readers should ready themselves by putting on shoes. In reconciling Jews and Gentiles into one body, Christ "is our peace" (2:14). The elimination of hostility through Jesus' death on the cross is central to the letter's understanding of the heart of the gospel message. It is this message of reconciliation that should lead the church to the behavior indicated here and in the rest of Ephesians 4-6.
In addition to these, the reader is exhorted to take up "the shield of faith" (6:16). According to Ephesians, faith activates the power of God (cf. 1:19; 2:8). Salvation is God's gift, yet it also comes through the believer's faith (3:12). It is "through faith" that Christ dwells in the believer's heart (3:17). Metaphorically, taking up the shield of faith communicates the protection that faith activates. The salvation that comes as God's gift through faith is depicted as the ability "to quench all the flaming arrows of the evil one" (6:16).
Prayer (6:18) is an activity that is connected to the taking up of God's armor. The author also prays on behalf of the church for their strength and understanding (cf. 3:13-19). The church is instructed to pray for all of the saints and for the author as well. The cosmic adversaries of 6:12 carry an eschatological tinge, because the imagery of God taking up God's armor to seek justice was related in first century culture to the notion of the day of the Lord. Yet in Ephesians' reworking of the imagery, the battle with cosmic forces is not simply a battle delayed for a future day of God's judgment, but is a present battle believers must engage on a regular basis.
The church's struggle is a heavenly one against spiritual powers, but it is acted out on a more mundane level in the types of behavior to which the reader is called. The "chains" of the writer's imprisonment (6:20) are another reminder of the ways that the "cosmic powers of this present darkness" impinge on the lives that believers live in this world. The armor of God does not mean that the church will not encounter difficulties, then, but enables Christians to encounter such difficulties. Through perseverance and prayer, the church may boldly proclaim the gospel even in the midst of persecution and hardship.