All that is in a full-grown tree was potentially in the seed. All that we find unfolded in the fulness of the gospel lies in a rudimental form in the earliest books of the Bible. What at first is only obscurely intimated is gradually unfolded in subsequent parts of the sacred volume, until the truth is revealed in its fulness. This is true of the doctrines of redemption; of the person and work of the Messiah, the promised seed of the woman; of the nature and office of the Holy Spirit; and of a future state beyond the grave. And this is specially true of the doctrine of the Trinity…
Therefore in the light of the doctrine of progressive revelation the Psalms, no matter how much they set forth Christ and redemption, can not do so in the full light of the New Testament. To argue otherwise is to reject this important principle of progressive revelation. To only sing Psalms is to sing that which contains less than the full light of New Testament teaching. The light of the Gospel gets brighter the further along in time we go, until in New Testament times we reach the fulness of that light. However the Psalter is part of the Old Testament revelation. As A. A. Hodge points out it can only in a 'rudimental' form set forth the gospel and that only 'obsurely'. Even the doctrine of God is not fully set forth in Old Testament revelation.
As Paul reminds us in the book of Hebrews this was an imperfect or incomplete system. If it was perfect and complete it would not have passed away and been replaced by a better system.
Though God commanded love in the Old Testament, yet the manner of giving the law bespoke more of fear than love. The dispensation of the law was with fire, thunder, etc proper to raise horror and benumb the spirit, which effect it had upon the Israelites, when they desired that God would speak no more to them. Grace is the genius of the gospel, proper to excite the affection of love. The law was given 'by the disposition of angels,' with signs to amaze; the gospel was ushered in with the songs of angels, composed of peace and good will, calculated to ravish the soul.
Instead of the terrible voice of the law, Do this and live; the comfortable voice of the gospel is, Grace, grace. Upon this account, the principle of the Old Testament was fear, and the worship often expressed by the fear of God; the principle of the New Testament is love. "The mount Sinai gendereth to bondage," Gal. 4:24; mount Zion, from whence the gospel or evangelical law goes forth, gendereth to liberty; and, therefore, the Spirit of bondage unto fear, as the property of the law, is opposed to the state of adoption, the principle of love, as the property of the gospel, Rom. 8:15; and therefore the worship of God, under the gospel or New Testament, is oftener expressed by love than fear, as proceeding from higher principles, and acting nobler passions. [Taken from his Sermon on Spiritual Worship based on John 4:24 and found in volume one of his works.]
1. The terms 'Psalms, Hymns and Spiritual Songs' inEphesians 5:19 & Colossians 3:16 do not refer to the 150 Psalms of the Book of Psalms.
2. Old Testament and New Testament saints did not live by this exclusive psalmist rule.
3. The angels and glorified saints did not sing the psalms.
4. There are possible remnants of hymns/canticles/doxologies quoted in the New Testament.