My understanding of the difference between substance and subsistence is:
- substance: an act of existence (being), with a certain essence (the definition of the thing, the quiddity or “what-ness”), and nature (having acts appropriate to the kind of thing that it is – it is the “nature” of a rabbit to be a herbivore)
- subsistence: a being whose act of existence does not inhere in another being; for example, an accident like the color red does not subsist even though it exists; something that subsists is not a quality or aspect of something else
In the case of Christ, there are two natures or substances: human and Divine. Each substance is it’s own nature (or, has it’s own nature in the case of the human substance), with their proper acts – acts proper to humanity and acts proper to the Divine substance. I believe this is the definition of what we call the “hypostatic union”, the one
hypostasis is the Son, uniting two natures.
What
subsists for Christ is the Second Person of the Divine Trinity: the Son. The Son subsists, and provides unity and is the basis of who we call Jesus Christ. So in this case, there are two substances/natures and one subsisting Being, the Son of God. There isn’t a separately subsisting human person and separately subsisting Divine Person in Christ – just one subsistence. For the rest of humanity, the one subsistence for the one human substance is the human soul.
In the matter of transubstantiation, prior to consecration, there are two separate substances: bread and wine respectively, subsisting in their own physical natures. After consecration, there is only one subsistent Being: the Son of God. The Son is present body, blood, soul and Divinity. Body, blood, soul are aspects of the human substance, while Divinity is obviously the Divine substance. The non-subsistent accidents (I believe from the Latin
accidere meaning to be a non-essential occurrence or characteristic) make the Lord appear to us to be bread and wine, but the substance(s) are the Lord’s human and Divine nature. It’s the Divine nature that allows the Lord to subsist in many places at one time.
I hope I haven’t said anything heretical.