NOTE: This is a condensed excerpt from my forthcoming (2014) commentary on Philippians.
When Paul uses the word peace it must be understood in light of its OT background. Peace (Heb. šālǒm, Grk. eirēnē) means not merely the cessation of hostility, but includes the ideas of wholeness and completeness. Often in the OT prophets it is a one-word shorthand for the resulting state of God’s eschatological salvation (e.g., Isa 32:15-18; 48:18; 52:7; 53:5; 54:10). Many of the NT authors adapt and develop this idea, including Paul. He uses eirēnēin a way that reveals the “already – not yet” dimension that is characteristic of much of his theology. So while believers already have peace as a result of God’s justifying decree (Rom 5:1), it is still something that must be pursued in their lives (Rom 12:18; 14:19; 2 Cor 13:11; Eph 4:3; 1 Thess 5:13; 2 Tim 2:22). Peace is both an aspect of the fruit of the Spirit (Gal 5:22) as well as the by-product of a mind set on the Spirit (Rom 8:6). But ultimate peace awaits the final day and as such forms part of the Christian hope (Rom 2:10). So far from being a stereotyped greeting, peace communicates the profound truth that we who once were enemies of God are now at peace with him through the blood of Jesus (Col 1:19-23).