1. Why Sing?
We must begin then by asking, Why sing in worship? The first and last records of singing in Scripture (Exod. 15:1; Rev. 15:3) have a link: redemption is the common theme - the church praising her deliverer, her Saviour. But more than example, we have command to sing: "Serve the Lord with gladness: come before his presence with singing" (Psa. 100:2).
2. Why Sing Psalms?
If we are to sing praise to God, then the immediate question which arises is, What are we to sing? As regards the Psalms there can be no doubt about their appointment for singing: "O come, let us sing unto the Lord...Let us...make a joyful noise unto him with psalms" (Psa. 95:1,2). The New Testament contains similar exhortations, especially those couched in the two controversial verses - Eph. 5:19; Col. 3:16. Churches which today effectively practise an "exclusive hymnody" should at the very least be asking themselves why God's hymnbook has NO place in their singing.
3. Why Sing Psalms Only?
The key texts here are: Eph. 5:18,19; Col. 3:16.
A. Believers are commanded to become filled with Christ's Spirit by the indwelling of His word.
B. This is to be accomplished by mutual instruction through singing.
C. Therefore the phrase "psalms and hymns and spiritual songs" denotes the Psalms of David, because they alone satisfy both of these conditions, i.e., they are the word of Christ and they are designed for singing...There is no record of Jesus or His apostles singing anything but psalms.
The psalms, hymns and songs are all said to be "spiritual" or "Spirit-given." Which of our hymn-writers would claim that for his writings? We are sure though that the Psalms are the very words of the Spirit and therefore the perfect vehicle to serve the purpose of singing in worship, which is, not the outpouring of our feelings, but the discipling of our souls.
-- Excerpted from David Blount in Why sing Psalms only?