Question #19: What does the singing of Psalms exclusively have to do with the closing of the canon? Is the writing of new songs the same as writing new Scripture?
Or we might even ask, “Is the canon really closed?”
There have been some recent questions on the EP website regarding the Canon of Scripture itself. I think this is related to the question of EP because it seems to me that those who write new songs are taking upon themselves the mantle of the Prophets. The authorized songs to sing in Scripture (which we would identify as the Psalms) were only written by inspired Prophets, and no one else. Why would we think we could write songs to God today when there is no office of songwriter in our age? Ours is an age when we recognize the completion and finality of God’s Word. This certainly implies that whatever inspired songs were written are no longer being written. Just as we accept God’s Word as being complete, so we accept God’s songbook as being complete. Anyone who writes a worship song in our day is doing so in direct opposition to the Bible itself.
Please post here defenses of the completion of the canon and questions regarding the authority of songwriters in our day.
Exclusive Psalmody 
Some recent discussion can be found on Question #9 found here
From The Psalms in Worship by John McNaugher:
“VIII. Another objection answered. Many hymns, it is said, were written during the century preceding the coming of Christ, and during His and the Apostolic days, which may well be used in the praise service. “The Psalms of Solomon,” a collection of eighteen psalms, and “The Songs of the Three Children” embodied in the Septuagint version of Daniel are cited as illustrations of this. It is said that the writing of psalms and hymns and songs after the Old Testament Canon was closed is proof that the Church may use them, and that she may write others for use. After the Old Testament Canon was closed there were many books written in the style of the Scriptures. First and Second Esdras, Tobit, Judith, additions to Esther, the Wisdom of Solomon, Ecclesiasticus, Baruch, First, Second, Third, and Fourth Maccabees, Enoch, etc., etc., are all of this type. They are religious books; they are similar in their contents to the books of the Bible; some of them are bound up in the Septuagint; and some of them are still in the Vulgate. But Protestants do not agree that they shall be used in our pulpits as Sacred Scripture. The same argument that would take the “Psalms of Solomon” and other hymns into the praise service of the Church would take all these apocryphal books into the reading service of the Church.”
THE SUITABLENESS AND SUFFICIENCY OF THE PSALTER FOR CHRISTIAN WORSHIP
By President J. A. Thompson, D. D., Tarkio, Mo.
“WHO questions the suitableness and sufficiency of the Psalter for Christian worship? The question is found in the sphere of the Church’s practice. A large part of the Christian Church has abandoned the use of the Psalter in the praise service; another large part makes small use of it. Dr. Archibald Alexander Hodge was accustomed to tell his classes that the United Presbyterian Church had been raised up in the providence of God to keep the Psalter before the Church. In the fulfilment of its destiny the United Presbyterian Church finds it necessary to champion the suitableness and sufficiency of the Psalter for Christian worship.
What is the formal worship of God? It is outward expression of an inward attitude toward God. It may assume several forms. It is seen in prayer, in praise, in preaching, in the sacraments, in giving, and in fasting. These forms may be divided into two general classes. In all cases the “form” of worship is prescribed in God’s Word. No one would think of attempting the formal worship of God in any way not prescribed in His Word. In prayer, in fasting, and in giving the “matter” is not prescribed other than in general terms. We may give to God anything that has value to us. We may fast for any one of a great variety of reasons. We may pray for any one of a multitude of things. “I will therefore that men pray everywhere, lifting up holy hands, without wrath and doubting” (i Tim. ii. 8). “Pray without ceasing. In everything give thanks: for this is the will of God in Christ Jesus concerning you” (i Thess. v. 17, 18). In the sacraments and in praise the ” matter” is prescribed to men. No orthodox Christian would think of substituting anything for the bread and wine which the Master used in ordaining His Supper. Whatever disputes Christians may have over the mode of Baptism, they do not dispute over the use of water as its symbol. In preaching we have the combination of the prescribed and the voluntary. It is in part instruction concerning God derived from His Word and in part testimony concerning our own experience of God. Praise belongs to that type of worship based entirely upon what God has revealed of Himself and of man. “Whoso offereth praise glorifieth Me” (Ps. 1. 23). God must reveal Himself as a foundation for praise. “O Lord, open Thou my lips; and my mouth shall show forth Thy praise” (Ps. li. 15).
There has been no change in the basic idea of praise. The eternal God remains the same. He is ” the Father of lights, with Whom is no variableness, neither shadow of turning.” Evangelical Christians agree in the belief that the Canon of revelation is closed. God has accomplished His purpose with men in revelation. There is no need for further revelation. The completion of the Canon has been definitely and finally declared. In completing the Canon God did not deem it necessary to add to the book of praise after the period of the Exile. Every feature of worship had been provided for in the Psalter. Christianity did not claim to be a new religion. It was the fruition of the religion of the Old Testament. The Jehovah of Israel’s prophets and the Father of our Lord are one and the same. Christianity has no new principle of praise worship to offer. It changed the form of the sacraments to meet new facts in the history of religion. It has given new vitality to prayer and clearer interpretation to praise. It has nothing to add to the character of God, nothing to tell of His relation to men which had not already been uttered by the Holy Ghost through the men whom He had inspired of old.”