Moderator: Brian raised side issues (musical instruments) which will not be discussed on this thread; so folks, please interact with the warrants He adduces in keeping with the topic and several moderator appeals throughout. Off topic and pointless posts will not be approved for posting. Feel free to refer back to previous posts instead of reinventing the wheel. For instance see Matthew Winzer's post on hymnic fragments as a basis of interaction; the matter of new songs generally has been roundly covered in the thread; but I do not recall if the argument from Revelation has been specifically addressed on this thread before now.
My view of the RPW is that those things that are prescribed in worship are in fact required. If they are required, even if not at all times, must be a part of worship at some time. That means that either EP is required, or it is prohibited. There is no middle ground in this issue as far as the RPW stands.
That means the debate here is critical for worship; it is NOT something which would define heresy ... those that "get it wrong" are not outside the church. It is not one of the signs of the true church. But it is not just what one prefers. Nor is it merely "what is allowed". If there is Biblical warrant to compose songs for worship that are not in the book of Psalms (or Songs, if we actually translate the word into modern English) then we have a command to use those songs. If on the other hand we have no warrant to do so, then it is wrong to worship God in that fashion.
Now, I also believe we should be tolerant on the issue toward those that differ, even if we believe them to truly be wrong.
My personal view is shaped from what I see the scripture state. The single most salient point being that we are given in fact a view of perfect worship in a perfect state. The worship of the saints in heaven through the revelation of Jesus Christ given by John. In that worship it is not Psalms alone that are used, but in several cases "a new song" that is not prior scripture. That being said, perfect worship (if the inspired Word recorded in the NT is correct) contains more than just the book of Psalms. Our worship is a reflection of what worship in heaven is (even as the OT worship was to reflect the worship of God in heaven -- even down to the Holy of Holies being a cube in shape, which is the same shape as the New Jerusalem). The commands Moses had to make sure everything fit the design revealed to him was to make sure what was produced was modeled after the true worship in heaven. That EP is not what is used in heaven ought be enough that it not even be thought of as a possibility for our worship.
Other items that speak against EP are that there are fragments of hymns used in the NT church that speak toward them being used, and yet no commands against their use ... while this is an argument from the negative, tied with the first, it becomes more powerful.
1 Cor 14:26 only makes sense in the context if it is speaking not about one of the OT Psalms, but new songs, and that is very clearly talking about the assembly of the saints.
As I said, the most salient is that perfect worship in heaven includes new and different songs. We are to worship, even in heaven, only as God commands (though I imagine that in heaven, we will not need to be "commanded" as our hearts will have his law written upon them). I would say what I believe is clear teaching is that we are to worship God with new music. That in heaven we use instruments (as we should here as well) and that what is done in perfection in the next age is done in reflection in this age.