Closed vs. Open System

A friend wrote to me recently that most clerics do not believe in an open universe vs. a closed one. In other words, they hold that there is nothing beyond the physical universe. While I don't think the problem is as big as my friend makes it, many clerics cannot find a reason to believe that the universe is not closed. This is a question which my metaphysic answers decisively, I think -- in the sense that it rules out a closed system. While philosophy can only say so much within the bounds of philosophy, I believe it is wrong to say that the universe is closed, and quite straightforward to prove this.