After a Catholic priest has done his transsubstantiation ritual, which forms affect the host and wine?
or does it depend on the priest, HIS inscrutable will?
note: It's a pure theory question, not game related, but I was wondering...
Those things cannot be affected by magic, so it does not matter. They are "divine" in substance, so no casting lousy spells at them. You can try, of course, I encourage you to do that. You are a powerful magus after all, so you must be able to affect them, right? 
Cheers,
Xavi
I agree with Xavi. After the transubstaniation ritual, the host and wine are the blood and flesh of Jesus. Jesus is not just a "supernatural creature" or even a spirit or being with a high Divine Might. Rather he is wholly God. God is completely above and immune to Magic. As such, I think the clear answer is that Hermetic magic (and any other magical, faerie or infernal power) is incapable of affecting the Host othe than by mundane handling.
That's not quite what Xavi said (and I agree with him). They become a divine substance and inscrutable to magic, preserving the mystery of the Eucharist. If they were immune to magic because they became God (as Christians believe), this would confirm the truth of Christianity and the falsehood of other Divine faiths such as Judaism and Catharism within Mythic Europe.
Of course, practically, the two are the same, except for the 'game truth' that the OP was theorising about.
Mark
I have to slightly disagree as it is possible for demonists to profane or desecrate (not sure which one is the correct word) the host in their impious rituals. I think there is even a reference to this in RoP:Infernal.
That might be magic, but not HERMETIC magic anymore.
CHeers,
Xavi
My take on this is that once the Host is defamed, it is no longer a part of the Divine, but part of the Infernal. I don't have RoP:I so I can't recall if the intact Host can be affected by Infernal powers before it is otherwise profaned. If so, then I think this is proof that (at least this much of) Christian doctrine is false, as the obverse of Mark Shirely's statement. I seem to recall the Infernal cannot affect the Host before it is profaned, but I admit that I haven't read RoP:I in a long time.
Of course, there is, as you note, no discernible differance between the Host truly becoming God, and the Host becoming something powerfully aligned with the Divine that defeats all attempts to distinguish it from God.
Which leads me to ask, as I don't recall, is there some discussion in RoP:D about keeping the truthiness of the Divine religions vague or ambigious? If the Host is merely made inscrutable to mortal magic, isn't that like God "hiding the ball," by making it appear that Christianity is correct? Wouldn't it at least be, in setting, a powerful argument that Christianity is correct?