When: Third Century

Where: Late Roman Empire

Central Error: That the three persons of the Holy Trinity were different "modes" or aspects of one God, rather than being distinct persons sharing one divine essence

Major Proponent: Sabellius (ca. 215)

Description: First taught by the priest and theologian Sabellius at Rome, Modalism (also known as Sabellianism after Sabellius or, in the East, as patripassianism (meaning "the Father suffered) taught that God was one divine being who presented himself in different "modes" or aspects, hence the name. This directly contradicts Scripture, which shows the three persons of the Holy Trinity interacting with each other (Matthew 3:16-17, Matthew 17:5, etc)

The heresy of modalism was vigorously opposed by the Father, especially Tertullian (in Against Praxeas and Hippolytus (Against Noetus and Philosophumena), and was eventually condemned by the Council of Rome, stating "We anathematize those also who follow the error of Sabellius in saying that the same one is both Father and Son”. (Tome of Pope Damasus, 2)

Modern Manifestations: The most well-known modern revival of modalism is the "Oneness Pentecostal" movement, such as the United Pentecostal Church. This movement takes a unique approach to its heretical teachings, teaching that "Jesus" is the proper name of the one divine being who only appears as the Father, Son and Holy Spirit.

Sources:
https://www.catholic.com/magazine/print-edition/sabellianism
https://en.wikipedia.org/wiki/Sabellianism