"My father painted like Cézanne and understood the southern French landscape the way Cézanne did. His vision of the world was sane, full of balance, full of veneration for structure, for the relations of masses and for all the circumstances that impress an individual identity on each created thing. His vision was religious and clean, and therefore his paintings were without decoration or superfluous comment, since a religious man respects the power of God's creation to bear witness for itself. My father was a very good artist." — Thomas Merton
Thomas Merton is a theological ninja, able to slip in undetected and drop an amazing thought before anyone realizes he's done it. It's what makes him both profound and fun to read. I mean, just read the quote again. See?
The idea that creation bears its own witness is critical for the religious-minded artist. At least, it should be. It would be easy for me to tear apart "Christian" art with this one idea/weapon. I won't speak for other religions here (you shouldn't do that anyway, generally), but I have enough life lived in the Christian experience to say that it's fraught with the desire to dress up what need not be, to create art that explains, rationalizes, and demystifies the divine life. Unfortunately, that's unnecessary at best and spiritually harmful at worst.
People want to make things which reflect their experiences in the world; that's what art is. In the process of shaping, molding, and recreating our encounter with the created, it's easy to get lost in the desire to pile on too much. We want to show others what we see, to drive them deep into the heart of what is or was meaningful to us. Merton is asking us to step back, to pause and let what is powerfully and beautifully created speak for itself. This doesn't mean we can't try to represent it in our own way. Rather, seeking to be a good artist, especially one with an eye towards the divine, means letting what is true hover as close to the surface of our work as possible. Quit covering up beautiful stuff, dammit.
When we equate being a good artist with "respecting the power of God's creation to bear witness for itself" as Merton does, we're giving ourselves a wonderful cosmology in which God infuses all that is, in which God is Being itself, calling the created world in such a way as to drive our participation in it. This is why trying to separate Christian art from the rest gets murky; it can all be Christian if it allows the Word an opportunity to speak clearly. It can all be religious. It can all be an expression of our being in the world, our belonging to Being (though just because it can doesn't mean it will). If it's good art, it'll tell us something true, something about what it means to be alive and engaged with a created world in which all things are becoming. And if it's really good art, it'll bear witness.