The scholastic scheme of the Trinity is ultimately modalistic. I hope to briefly shed light on this here.
The scholastic scheme of the Trinity, following Augustine, the Fourth Lateran Council, and medieval scholastic thought, can be summed up as follows: There is one simple Supreme Being, the one God, Who is Father, Son, and Holy Spirit. The Father, Son, and Holy Spirit are ‘persons’ within this Supreme Being, a ‘person’ not being what is normally meant by the word ‘person’, but rather ‘a mode of subsistence’. These three modes of subsistence within the Supreme Reality are individually co-essential (sharing the same individual numerical being) but are distinguished by their causal relations, namely that the Father is unbegotten, the Son begotten from the Father, and the Spirit proceeding from the Father and the Son. The ‘persons’ share one will, mind, consciousness, etc, as these are all proper to the Supreme Being Himself, rather than the ‘persons’ within (ad intra) the Supreme Being.
The problems with this view are many. Firstly, making the Supreme Being both caused and uncaused (even if that causality is ad intra, not ad extra) is to deny the first principle of natural theology, that there is one Supreme Being Who is the uncaused Cause of all else that exists. While the scholastic view maintains that the Supreme Being the is Cause of all else that exists, they are unable to confess the Supreme Being to be wholly uncaused, as the subsistences of the Son and Spirit are caused, and are only distinguished within the Supreme Being by those causal relations.
Secondly, this is ultimately pure modalism, dressed up a bit to sound trinitarian. Because the Thomistic doctrine of divine simplicity says that everything within God is identical to itself, there is no ability for true distinction with the Supreme Being. God’s love and God’s wrath, for example, due to divine simplicity, are said to be the same thing, each being coterminous with the other. While this may sound relatively innocuous when applied to attributes, logically, since it applies to all that is within the Supreme Being, it must also extend to the ‘persons’ within the Supreme Being as well. Thus, just as much as God’s attributes are not truly distinct but all one and the same thing, merely talked about under different names for our convenience, so to the ‘persons’ of Father, Son, and Holy Spirit must be individually identical to one another, and so, not truly distinct at all, only spoken of under different names. The causal relations that are supposed to distinguish the persons are incapable of doing so, because these causal relations are all ‘ad intra’, that is, internal to the Supreme Being, where all things are identical to one another according to Thomistic simplicity. Thus the causal relations must logically be identical to one another, and therefore wholly fail to distinguish the persons.
This modalism can also be seen another way: each ‘person’ is said to be coterminous with not a part of, but the entirety of, the Supreme Being. Thus, each person is individually equal to the same Supreme Being; and thus, each person must be individually equated with one another. For if the Father is the whole Being, and the Son is the same whole Being, They are equated with one another, on the logic of ‘If A=C, and B=C, then A=B’. Scholastic ‘trinitarianism’, then, is in fact not trinitarian at all, but modalism, only slightly modified from that of Sabellius and Noetus, at the end of the day.
Thirdly, the ‘essence’ shared by the ‘persons’ is actually a person, while the ‘persons’ are not. That is, the Supreme Being, in which Father, Son, and Spirit are supposed to subsist, meets the actual definition of a ‘person’, which is a rational individual being. But the Father, Son, and Spirit, being only modes of subsistences within that Supreme Being, are not individuated, and so, are not persons, according to the standard definition of the term. To put scholastic conceptions of the Trinity into 21st century language, then, would be to state a belief in ‘one person with three internal modes of being’. This is modalism.
Having shown, then, that the scholastic scheme of the doctrine of the Trinity is modalism, then, it is sufficiently refuted by that fact. It fails to live up to the name ‘trinitarianism’, and falls into all of the same damning errors as modalism. Ultimately it is all the same individual Supreme Being, the same person, according to the normal usage of ‘person’, Who created the world and died on the cross and rose from the dead. There is no Father and Son; the Supreme Being has no eternal Son, and so the existence of the Son is denied outright, and the Supreme Being, Who we would otherwise call Father, is the one who died on the cross. The thing called ‘Son’ is merely a mode within this Supreme Being, this sole person, and not in fact the Son of God, but merely a mode within God, incapable of being called ‘the Son of God’ with any truthfulness.
The solution to this long-standing problem is to reject scholastic ‘trinitarianism’ as the modalistic heresy it is, and believe in the Supreme Being, the one God, as scripture and natural theology would have us, as the one supreme uncaused Cause of all else that exists, as the Father alone (for He alone is uncaused); not believing that is one Supreme Being, the Father, is alone and solitary, but that He eternally has His only-begotten Son with Him, Who He begat from Himself before creation, as a distinct (really existing) individual being besides Himself, through Whom the Supreme Being created all things, rules over all things, and reconciles all things to Himself; and in one Holy Spirit, a third distinct individual being or person. We must return to and embrace a biblical (and, incidentally, patristic) doctrine of the Trinity; believing not that the Supreme Being is somehow Father, Son, and Spirit Himself, but believing, as the scriptures teach, that the Supreme Being is in truth Father to a Son, another distinct individual being besides Himself.