‘Semi-modalism’ is the false doctrine that teaches that the three real persons of the Trinity are together a single person. Most semi-modalists, however, refuse to use the term “person” for the Trinity, although conceptually they treat the Trinity as a whole as a person in every way except using that term for it.
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.