The great themes of the Psalms are, Christ, Jehovah, the Law, Creation, the future of Israel, and the exercises of the renewed heart in suffering, in joy, in perplexity. The promises of the Psalms are primarily Jewish, and suited to a people under the law, but are spiritually true in Christian experience also, in the sense that they disclose the mind of God, and the exercises of His heart toward those who are perplexed, afflicted, or cast down.
The imprecatory Psalms are the cry of the oppressed in Israel for justice--a cry appropriate and right in the earthly people of God, and based upon a distinct promise in the Abrahamic Covenant ((See Scofield "Genesis 15:18") ), but a cry unsuited to the church, a heavenly people who have taken their place with a rejected and crucified Christ. ( Luke 9:52-55 ).
The Psalms are in five books, each ending in a doxology: