The Epistles are distinguished from the Gospels in that they are not narrative compositions. In terms of redemptive history, they are written on the other side of the cross and resurrection, so that they typically reflect more deeply on the significance of Christ's death and resurrection than the Gospels do. The implications of the fulfillment of God's promises in Jesus Christ are explored and applied to the readers in the Epistles. These same themes are present in the Gospels, of course, but they are not set forth in the same fullness, since the nature of Jesus' messianic mission often perplexed his disciples during his earthly ministry, and they grasped these realities in their fullness (though still not exhaustively!) only after the cross and resurrection and with the outpouring of the Spirit at Pentecost. The Epistles have played a major role in the formation of doctrine and Christian theology throughout church history precisely because they expound on the great themes of God's saving work on the cross. Because they reflect on and explain the fulfillment of God's promises in light of the OT and the Gospels, it is particularly fruitful to study their use of the OT, OT allusions, and citations of and allusions to Jesus' teachings. By doing this we understand more clearly how epistolary writers understood the fulfillment of God's promises in Christ. We also perceive how they related the OT and the gospel traditions to the churches, and such an understanding assists us in applying not only the Epistles but also the OT and the Gospels to today's world.
Among the major themes in the Epistles are the following: (1) Jesus Christ is the fulfillment of God's promises in redemptive history. He is Messiah, Lord, the Son of God, and the true revelation of God. (2) The new life of believers is a gift of God, anchored in the cross and empowered by the Holy Spirit. (3) Christians experience salvation by faith, and faith expresses itself in a transformed life. The Epistles spend considerable space elaborating on believers' newness of life. (4) Believers belong to the restored Israel, the church of Jesus Christ, which must live out her calling as God's people in a sinful world. (5) In this present evil age believers suffer affliction and persecution, but they look forward with joy to the coming of Jesus Christ and the consummation of their salvation. (6) False teachers dangerously subvert the true gospel of Christ.
Taken from the ESV Study Bible copyright ©(2008). Used by permission of Crossway, a publishing ministry of Good News Publishers, Wheaton, Il 60187, www.crossway.org.