I'm going to get this down on electron because I've been thinking about it for a while, but I'm not yet 100% serious about all this.
I think the main undiagnosed heresy of the West is Nestorianism. In fact, I suspect most of us are crypto-Nestorians. I'm taking the essence of Nestorianism to be exemplified by the idea that in Jesus Christ there was not a unity of two natures in one person but rather that the two persons (the Son of God and the man Jesus) with the two different natures were in perfect "sync". (I'm completely bracketing the question of how Nestorian was Nestorius, mostly because I don't care. His heresy is a pretty good example, I think, for a way of thinking that is, I suspect, rife among otherwise orthodox Christians.)
Among Catholics, I think the most obvious manifestation comes when we think about the power of the sacraments. I suspect that many self-described orthodox Catholics, when they think of the power of the sacraments, do not think of the sacraments as actually, in themselves, accomplishing anything, but rather they believe in a kind of parallelism between the natural effects of the sacrament and the power of God, such that while the priest is, say, pouring the water, God is, at the very same time - and in perfect sync with the priest - cleansing the soul, but that the spiritual movement by which God cleanses the soul and the physical movement by which the priest pours the water are totally different and that there is no real relationship between them other than coincidence and exemplification. I suspect most Catholics think of the real power of the sacraments as lying "underneath" the visible, material thing but not being "in" it. Or else being "in" it like an egg is in a box, not "in" it like the meaning of a word is "in" an arrangement of letters.
I'm pretty sure all Protestants think this way, and I suspect this shared understanding lies behind almost all arguments on the sacraments. Everybody is thinking of the sacraments like this. The Protestants denying this, while Catholics affirm it. The funny thing is that I think the Protestants are right to deny it, and the Catholics are wrong to affirm it. But the Protestants have not escaped heresy. They're still wrong, because they just deny any relationship at all between the working of God and the sacraments.
- Were the feet of Mother Teresa, as wrinkled, gnarled, and rough as they were, really beautiful? Can the twisting of nature communicate a form of beauty that goes absolutely beyond nature itself?
- Our inability to believe in the goodness of nature, in the ability of nature to communicate anything supernatural, is behind this.
- A clue to all this is understanding the power of the sacraments as being genuinely instrumental. The sacraments are instrumental causes of grace. But I suspect we don't believe that their naturalness, their natural powers, have anything to do with this.