God in His infinite wisdom and mercy has provided everything for His worship. He has not left us to guess as to how to worship Him. God can only be worshiped according to what He has commanded in Scripture. As we confess in the Westminster Shorter Catechism, “The word of God which is contained in the Scriptures of the Old and New Testaments is the only rule to direct us how we may glorify and enjoy Him.”
In that answer to question 2 of the catechism, we see the reformation teaching of Sola Scriptura. The doctrine of Sola Scriptura was not invented during the reformation but is the teaching of the Scriptures themselves. In Deuteronomy and Revelation especially we are warned not to add to or subtract from the Scriptures. The penalties for doing so are severe, and we must fear God and take His warnings with the greatest of seriousness.
The infallible, inerrant and perfect Holy Word of God totally is unique and of a different sort than all other writing.
For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts. Isaiah 55:9. God through Isaiah is not merely pointing out that His ways and thoughts are only higher than ours, but is telling us they are infinitely higher. They are in a different category altogether. Accordingly, the Scriptures are different because they were breathed out by God the Holy Ghost, the Spirit of Christ, carrying along the holy men who wrote them down. The Scriptures are different and unique because of they are the thoughts of God He chose to communicate to us and are the Word of God. Because they come to us by the agency of God the Holy Spirit, having been breathed out by God the Holy Ghost, the Scriptures bear His mark though and though. Every book, sentence and word of the Scriptures exhibit their being the very Word of God.
So what does that have to do with Exclusive Psalmody? Everything!
The very simple and plain truth is that the Psalms are scripture and other songs are not.
In Ephesians 5:18, 19
And be not drunk with wine, wherein is excess; but be filled with the Spirit; Speaking to yourselves in psalms and hymns and spiritual songs, singing and making melody in your heart to the Lord;
We are told and commanded to be filled with the Spirit, and how to be filled with the Spirit. We are filled with the Spirit when we sing “psalms, hymns and spiritual songs.” Many will say – see – God commands the singing hymns. Case closed.
Not so fast.
God does command us to sing psalms and hymns and spiritual songs in order to be filled with His Spirit, that is God the Holy Ghost. We need to ask, what kind of hymns and spiritual songs will fill us with His Spirit? Certainly we all agree, that Top-40 hits, Rock ‘n Roll or Jazz, to name but a few, are not the kinds of songs that would fill us with the Spirit.
However, perhaps songs we call spiritual songs can fill us with the Spirit. Perhaps songs about God, and Christ and his work we or others have written can fill us with the Spirit. After all they are about things we consider to be spiritual. There are many books filled with beautiful songs about Christ. Surely these can fill us with the Spirit, but can they?
The answer is obvious, but foreign to our natural minds and hearts. The answer is no.
The only hymns and spiritual songs that can fill us with the Spirit, God the Holy Spirit, are those hymns and songs that were breathed out by Him.
Absolutely no songs other than those found in the Scriptures can fill us with God the Holy Spirit. The only way to be obedient to the command of Ephesian 5:18,19 is to sing songs from the the Scriptures. The hymns and songs not breathed out by God the Holy Spirit cannot fill us with Him. A man cannot become drunk by drinking water, and neither can he filled with the Spirit by singing hymns and songs not breathed out by God the Holy Spirit. Just as there is no alcohol in water to intoxicate us, there is no God the Holy Spirit to be found in songs outside of those in the Scriptures.
There is no commonality between the psalms, hymns and spiritual songs breathed out by God the Holy Spirit found in the Scriptures and psalms, hymns and spiritual songs written by ourselves or others. Because they are inspired, breathed out by God the Holy Spirit, the psalms, hymns and spiritual songs found in the book of Psalms in the Scripture are completely different than all the others written by mere men not carried along by God.
This is not hard to understand. It’s easy.
What to do?
Don’t fill yourselves with empty hymns and songs devoid of the Holy Spirit. Sing the Songs of God the Holy Spirit found only in the Scriptures, for only by them will the singing fill you with the Spirit, God the Holy Spirit, the Spirit of Christ Jesus.
This is one reason why we sing the Psalms exclusively.