"Traditional" usually means something like, how many years of dust have piled up on the manuscript. But it's not a question at all of how many but of what kind. Something is traditional because of the tradition it plugs itself into. On the other hand, contemporary usually means (especially to the traditionalist) new and, hence, cheap. But this is misguided as well. This can all be illustrated by the simple fact that J. S. Bach was contemporary when he was writing his music. Who says it isn't traditional?
Once we release ourselves from this false conception of the traditional and the contemporary, the whole situation becomes much simpler. The Scriptural mandate for "new songs" and contemporary music is inescapable. It now depends upon what kind of contemporary music. This necessitates a method, a theory, an education, an analysis, a science, and a tradition of its own that Church composers can plug themselves into. This will make them simultaneously traditional and contemporary composers.
So, you ask, that's all very nice, but what's the concrete side of this? What musical science and analysis and theory and education and tradition should we be composing new music in? The answer is obvious - a culture, with its worldviews, doctrines, and presuppositions, that can produce music like Josquin, Dufay, and Perotinus is one that must have a coherent, methodological approach to it. Why else would he be called Magister Perotinus? He teaches something. All we have to do is teach it once again.
One more comment on tradition and the qualitative definition. As much as it is a qualitative sort of thing, tradition is inescapable. No matter how revolutionary your music is, it is always in some sort of tradition. As much as many people would like to think ex nihilo Schoenberg fit, even he can be traced through Wagner through Beethoven through Bach, all the way back to Magister Perotinus. You may digress from tradition and you may even revolt from it, but in so doing, not only are you still being affected by it, but all that you'll do is revolt into a different tradition. It's still always tradition, though. This holds true for Schoenberg and for Zep.
So, it becomes even more important when we approach what usually passes for worship music in, say, PCA, OPC, PCUSA, and all those other Reformed denominations. What tradition is your music plugging itself into? What culture does it sound like it's coming out of? Does it sound like, if you took away the words, it would be a bad imitation of a mild (or not so mild) pop song? It should be terrifying to us that our music is always traditional when we analyze what tradition it came out of.