This book has been called the heart of the Bible. It expresses sentiments produced by the Spirit of Christ, whether of prayer, sorrow, confession, or praise, in the hearts of God’s people, in which the ways of God are developed, and become known, with their blessed issue, to the faithful. The book is distinctly prophetic in character, the period covered by the language of the Psalms extending from the rejection of Christ (Psa. 2; Acts 4:25-28) to the Hallelujahs consequent on the establishment of the kingdom. The writers do not merely relate what others did and felt, but expressed what was passing through their own souls. And yet their language is not simply what they felt, but that of the Spirit of Christ that spoke in them, as taking part in the afflictions, the griefs, and the joys of God’s people in every phase of their experience. This accounts for Christ being found throughout the Psalms: some refer exclusively to Him, as Psalm 22; in others (though the language is that of the remnant of His people), Christ takes His place with them, making their sufferings His sufferings, and their sorrows His sorrows. In no part of scripture is the inner life of the Lord Jesus disclosed as in the Psalms.
From Concise Bible Dictionary