I thought the response I wrote might be of interest and help to some readers. I would argue that, whatever other issues one might have with substance dualism, this position is not ruled out dogmatically. (This is not to say that I endorse the position or that I even consider it reasonable, of course.)
The ratified decree of the Council of Vienne, promulgated by the Apostolic See, says this:
Moreover, with the approval of the said council, we reject as erroneous and contrary to the truth of the catholic faith every doctrine or proposition rashly asserting that the substance of the rational or intellectual soul is not of itself and essentially the form of the human body, or casting doubt on this matter. In order that all may know the truth of the faith in its purity and all error may be excluded, we define that anyone who presumes henceforth to assert defend or hold stubbornly that the rational or intellectual soul is not the form of the human body of itself and essentially, is to be considered a heretic.Here is the answer I gave to the questioner, which I've adapted from our exchange:
"It has been a while since I investigated this, but yes, I was pretty convinced at the time that the condemnation does not rule out substance dualism. Here are a few reasons:
1) The text itself just says that the intellectual soul is per se and essentially the [substantial] form of the human body. That's what it says. That statement is not equivalent to the statement that a substantial form cannot be a substance. Someone might personally think that follows, because of a certain metaphysics of substantial forms. But the Council itself does not assert that.
(Incidentally, as far as the metaphysics goes, Aristotle himself says that the form is a substance, in Metaphysics. The interpretation of these texts is quite convoluted of course, but it's clearly there. I'm not saying Aristotle was a substance dualist, of course.)