When doctrinal error is mentioned in respect to the Fourth Lateran Council, a number of issues could be brought up depending on what tradition is examining the council. Protestants reject its teaching on transubstantiation as error; Eastern Orthodox reject its teaching on the Filoque; the Oriental Orthodox would reject its Chalcedonian articulation of the hypostatic union. Everyone but the papists themselves takes issue with the council’s strong assertion of papal supremacy and authority (written, conveniently, by the Pope himself, as all the canons). But in this article, I want to draw attention to a lesser-known doctrinal error the council did much to promote: the anti-trinitarian doctrine of semi-modalism.
The Fourth Lateran Council is not primarily known today for its decisions regarding the doctrine of the Trinity. The thirteenth-century Papal Council, held in a Roman palace, dealt with a host of issues, including crusades, defining and officially confessing the doctrine of transubstantiation, the filioque, and papal authority. Yet its impact on trinitarian doctrine for Roman Catholicism is actually very great (the council is generally rejected by protestant and Eastern churches, as it took place after the Great Schism, and prior to the Reformation, with significant parts of its rulings being rejected by the Reformers).
The council’s importance to Rome’s views on the Trinity is primarily because of the council’s dealings with Abbot Joachim of Fiore’s treatise on the Trinity, in which Joachim accused Peter Lombard of teaching heresy in his famous Sentences. The heresy Joachim had in view was none other than semi-modalism. Abbot Joachim recognised that teaching that the Trinity was a single conscious thing who is the three real persons of Father, Son, and Spirit, is far different than scripture’s teaching of that the one God is only one person, the Father, and made efforts to draw attention to this departure from scripture’s teaching. He correctly pointed out that Peter Lombard’s semi-modalism effectively made the Trinity itself into a fourth divine person, ultimately to the destruction of the doctrine of the Trinity.
The bishop of Rome and the council he had called did not agree with Abbot Joachim’s assessment. His teachings on the subject were condemned, and the council affirmed the already well-entrenched heresy of semi-modalism as the official Roman Catholic belief, as they officially redefined the doctrine of consubstantiality to no longer teach that the Father, Son, and Holy Spirit share one and the same divine nature to instead mean than the Father, Son, and Spirit were the same conscious “reality”- in concept, a person. They avoided the language of “person” for this reality, denying Abbot Joachim’s criticism that conceiving of the Trinity this way was to believe in a fourth person of the Trinity, since to admit such would be obviously heretical.
This equivocation on the terminology of “person” and on the subject of consubstantiality have continued down to our own day, as semi-modalists continue to follow the pattern of substituting out another word besides “person” for the singular, personal, conscious, rational reality that they teach is the three persons of the Father, Son, and Spirit. They call this “thing” the “essence” or “substance” which exists in the three persons of the Trinity, while originally the Nicene church fathers introduced this language not to indicate that a person was three persons, but to speak of the single divine nature shared by the three persons of the Trinity. This fact can be seen clearly from their own writings.
Hilary of Poitiers, for example, wrote thus:
If any one dares to say that the Unborn God, or a part of Him, was born of Mary: let him be anathema.
42. The fact of the essence declared to be one in the Father and the Son having one name on account of their similarity of nature seemed to offer an opportunity to heretics to declare that the Unborn God, or a part of Him, was born of Mary. The danger was met by the wholesome resolution that he who declared this should be anathema. For the unity of the name which religion employs and which is based on the exact similarity of their natural essence, has not repudiated the Person of the begotten essence so as to represent, under cover of the unity of name, that the substance of God is singular and undifferentiated because we predicate one name for the essence of each, that is, predicate one God, on account of the exactly similar substance of the undivided nature in each Person.” (De Synodis)
Even in the post-nicene period, this classical understanding of co-essentiality can be clearly seen in the Chalcedonian Definition when it says:
“We, then, following the holy Fathers, all with one consent, teach men to confess one and the same Son, our Lord Jesus Christ, the same perfect in divine nature and also perfect in human nature; truly God and truly man, of a rational soul and body; co-essential with the Father according to the divine nature, and co-essential with us according to the human nature; in all things like unto us, without sin; begotten before all ages of the Father according to the divine nature, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the human nature…”
Notice Christ is said to be co-essential with man according to his human nature. This is consistent with an understanding of essence as a general nature considered in abstract, such as human nature, or the divine nature. Christ being co-essential with man literally means he is of the same human nature as all other men. By way of parallel, which is obviously drawn by the Definition, Christ is also eternally co-essential with the Father as His Son, in that He has from all eternity the same divine nature as the Father. This same understanding can also be seen articulated by Basil the Great (see: https://contramodalism.com/2018/01/12/basil-the-great-on-the-distinction-between-essence-and-person/ ).
In contrast, the idea that co-essentiality would somehow mean that the subjects were one “thing”, with its own real concrete existence, does not fit at all with the Chalcedonian Definition. Christ is co-essential with man- yet there is no real existence to the human nature considered in abstract. Human nature finds real existence in human persons; but considered in abstract, it is only an idea, lacking concrete existence. Yet if we apply the Fourth Lateran Council’s semi-modalistic re-definition of co-essentiality to the Chalcedonian Definition, this is exactly the way we must understand it. Yet clearly, this idea is nonsensical.
So we are able to see a medieval papal redefinition of co-essentiality:
“We, however, with the approval of this sacred and universal council, believe and confess with Peter Lombard that there exists a certain supreme reality, incomprehensible and ineffable, which truly is the Father and the Son and the holy Spirit, the three persons together and each one of them separately. Therefore in God there is only a Trinity, not a quaternity, since each of the three persons is that reality — that is to say substance, essence or divine nature-which alone is the principle of all things, besides which no other principle can be found. This reality neither begets nor is begotten nor proceeds; the Father begets, the Son is begotten and the holy Spirit proceeds.” (From Canon 2)
This redefinition of co-essentiality is erroneous, as it ultimately makes the Father, Son, and Spirit into a single person who is all three together. This doctrine is mutually exclusive to the classical doctrine of the Trinity taught by scripture and the orthodox church fathers of the Nicene and Ante-Nicene eras which is summed up in the Nicene Creed.
The Roman Catholic Church needs to abandon this grievous error and return to the classical trinitarianism contended for by such Western church fathers as Irenaeus of Lyons and Novatian. Those of other traditions should take heed of this error hidden among the historically more conspicuous problems with the rulings of the Fourth Lateran Council. We may be thankful that both Protestant and Eastern churches are free from commitment to the canons of this council, and thus are not, like the Roman Catholic church, bound to the heresy of semi-modalism in an official capacity by the ruling of the Papal council.