Exclusive Psalmody. They argue that the Biblical Law of Worship demands that we not worship God in any other way than what is prescribed in the Scriptures, therefore we may not sing anything in worship but the 150 Psalms of David. This position is based upon a number of anachronistic presuppositions and reflects a wrong view of "worship" in general.
Those who are engaged in this debate are more or less conservative Presbyterians (and Presbyterian wannabe Reformed folks) who have assumed a certain format for "worship" and are debating whether or not what gets plugged into that format should consist of Psalms only or Psalms and man-made hymns. Both pro- and anti-Exclusive Psalmody Presbyterians (as well as most evangelicals) would understand "worship" to consist of meeting with fellow believers, singing some songs, being led in prayer, hearing the Scriptures read, and hearing a sermon. What is sung is the psalms and/or hymns. The rest is not sung. The Exclusive Psalmodist does not argue that the sermon should be a verbatim recitation of one of the Epistles (which are inspired sermons given to us by God). Nor does the Exclusive Psalmodist say that we must only pray the prayers which are given to us in the Scriptures (which there are plenty of in the Bible). It is only what is sung according to the Exclusive Psalmodist that must conform to the Biblical hymnbook contained in the book of Psalms.
But this is problematic for a number of reasons. Historically, did the Christian church sing? Yes and no. What most people would understand as singing today did not get introduced into the church until the late middle ages. If you were in the church at the time of the Apostles you would not hear anything like "Amazing Grace." You would hear people engaging in a form of chant. Christian worship combined the worship of the synagogues with the worship of the temple and gave it a Christian twist. In Christian worship the book of Psalms has always played an important role. A Psalms was chanted as the celebrant entered. But the Psalm was not the only thing chanted. The prayers were chanted and the Scriptures were chanted rather than just read. Chanting served several very important functions. Chanting aids in memorization. Some have managed to decipher the chant notation found in the Old Testament. Much of Old Testament Scripture most likely was passed down orally through chant prior to being written down. Chant also kept the reader of Scripture from imposing a particular interpretation on the text through dramatic reading. But the Psalms were among various things that were chanted during the service which included prayers, the Scriptures and liturgical texts. There was not the sharp dividing line that we see today in the modern evangelical church between prayer and song. Throughout most of church history, hymns were not sung during the Divine Service but liturgical texts, antiphones, and responsories were all part of the service. Hymns were sung during Matins and Vespers. These were also chanted. Although the Puritans may have desired a more Biblical form of worship, what they ended up with was a type of worship that nobody had ever seen before. Throughout the Book of Revelation we find liturgical worship taking place and hints at liturgical worship throughout the Gospels and Epistles.
But there is a fundamentally deeper problem when people start speaking of the "Biblical Law of worship." True worship is Gospel. True worship is Divine Service. True worship is God serving us, not us serving God. True worship is not giving something to God and showing that I have fulfilled the Law. True worship is receiving God's good gifts through faith. All the complexities of the Old Testament Laws for worship were designed to show the Israelites that it was impossible for them to have a right relationship with God by worshipping Him rightly. They would always fail and so do we. The true God as He has revealed Himself in Christ is not like the pagan gods. He doesn't need us to feed Him food and He does not need us to feed Him glory. In the book of Acts, the early Christians met to "break bread" which is a reference to the Lord's Supper. That was the reason for their gathering. They met to receive. They met to receive Christ's body and blood. That was the purpose of the assembly.
Nobody who reads the 10 commandments on his own or guided by the historic teachings of the church is going to reach the conclusion that they are teaching exclusive Psalmody. There are plenty of real sins contained within the real 10 commandments that I break every day. I don't need to wedge in new commandments. The creation of new commandments to bind the consciences of others is idolatry--it makes the individual's brain his god. He plays Biblical hopscotch and finds laws through strange logical inferences and then condemns others for not following these laws. It becomes a way to distract himself from his own real sins and think that he's a little bit better than that guy who sings hymns.
If exclusive Psalmody is not the answer, should what happens when we gather in the name of Christ be a free-for-all? I don't think so. It would seem that the best solution would be to let what true worship is determine how we worship. If worship was all about us feeding God some glory then maranatha praise choruses or exclusive psalmody might work. If worship is all about emotional manipulation and our feelings then some post 1750 hymns or praise choruses might do the trick. But if true worship is not even really worship but a Divine Service where we receive God's good gifts by faith then we must look elsewhere. Do we need to invent something completely new to express this in the best possible way? I really don't think so. Instead, we should join in the historic liturgy that has existed in some form since the time of the Apostles and which most Christians still use. The historic is all about Christ and what He has done for us.
What does that look like? Some of it can be heard here but I will try to summarize it. All of the sections are drawn from the Scriptures. We begin by confessing that we are sinners. We receive forgiveness for our sins. Then a Psalm is chanted (Introit). We sing the Gloria Patri to the Triune God showing that our understanding of the Psalm is different from that of the Jews. Then we call upon Christ for mercy. Then we sing the song the angels sang when they announced Christ's birth. Then we pray. There is an Old Testament Reading. Then we chant a short portion of the Scriptures. Then there is an Epistle Reading. Then we stand as the Gospel Book is brought out and sing Alleluia. We sing praise to Christ. We say the Nicene Creed which is focused upon Christ and His work. We sing a hymn. The pastor preaches a sermon that shows us that we are real sinners and then tells us that Christ died for our sins and that we are forgiven. As the bread and wine are brought out we sing and ask God to cleanse our hearts. Then there is the prayer of the church. In the words of Isaiah we sing to the "Lord God of Sabaoth." We pray the Lord's Prayer. The pastor chants the Words of Institution and pronounces the peace of the Lord on us. We ask the Lamb of God who takes away the sin of the world to have mercy on us and grant us His peace. Then we kneel for communion. The pastor places the body of Christ on our tongues (showing we do nothing but receive the good gifts) and says "The true body of Christ given for you." He gives us Christ's blood and says, "The true blood of Christ, shed for you." Then we sing the Song of Simeon, we thank God, and we are blessed with the Aaronic blessing.