By the 4th century, a consensus had developed in favor of Trinitarianism, and modalism was generally considered a heresy.
Modalistic Monarchianism is accepted within Oneness Pentecostalism. Much of the movement's theology attempts to begin with an Old Testament understanding of God in order to understand what the first apostles would have believed about Jesus. It also seeks to avoid use of theological categories produced by Platonic-Aristotelian epistemologies, preferring rather to tell the story of redemption through narrative. Thus, the distinction found in the New Testament writers between God the Father and Jesus is understood to be from the attempts to identify God the Father and Jesus together, rather than to separate them more than necessary.
Bernard, David (1993). "Father, Son, and Holy Ghost". The Oneness of God. Word Aflame Press. ISBN 978-0-912315-12-6. Archived from the original on February 9, 2008. The Bible speaks of Father, Son, and Holy Ghost as different manifestations, roles, modes, titles, attributes, relationships to man, or functions of the one God, but it does not refer to Father, Son, and Holy Ghost as three persons, personalities, wills, minds, or Gods. God is the Father of us all and in a unique way the Father of the man Jesus Christ. God manifested himself in flesh in the person of Jesus Christ, called the Son of God. God is also called the Holy Spirit, which emphasizes his activity in the lives and affairs of mankind. God is not limited to these three manifestations; however, in the glorious revelation of the one God, the New Testament does not deviate from the strict monotheism of the Old Testament. Rather, the Bible presents Jesus as the Father, the Son, and the Holy Ghost. Jesus is not just the manifestation of one of three persons in the Godhead, but he is the incarnation of the Father, the Jehovah of the Old Testament. Truly, in Jesus dwells all the fulness of the Godhead bodily. 978-0-912315-12-6
"Definition of Modalistic Monarchianism". Merriam-Webster. Merriam-Webster, Inc. Retrieved 2021-10-16. Monarchianism holding that Jesus Christ was not a distinct person of Trinitarianism but was rather one of three successive modes or manifestations of God https://www.merriam-webster.com/dictionary/Modalistic+Monarchianism
"Monarchianism | Christianity". Encyclopedia Britannica. Retrieved 2021-10-16. Modalistic Monarchianism took exception to the "subordinationism" of some of the Church Fathers and maintained that the names Father and Son were only different designations of the same subject, the one God, who "with reference to the relations in which He had previously stood to the world is called the Father, but in reference to his appearance in humanity is called the Son." It was taught by Praxeas, a priest from Asia Minor, in Rome about 206 and was opposed by Tertullian in the tract Adversus Praxean (c. 213), an important contribution to the doctrine of the Trinity. https://www.britannica.com/topic/Monarchianism
"Sabellianism". Catholic Answers. Retrieved 2021-10-16. In 382 the Council of Rome, with Pope Damasus I presiding, condemned the heresy, 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). https://www.catholic.com/magazine/print-edition/sabellianism
"Sabellianism". Banner of Truth USA. 2016-05-18. Retrieved 2021-10-16. The revelations of Father and Son therefore, to Sabellius, belonged to the past, and the Church now was the Church of the Spirit, and after the end of the age, there would just be God, who would be neither Father, Son, nor Spirit. His teaching was rightly condemned by the Church, which understood that it strikes at the very foundations of Christianity. https://banneroftruth.org/us/resources/articles/2016/sabellianism/
McGrath 2013, p. 56. - McGrath, Alister E. (2013). Historical Theology: An Introduction to the History of Christian Thought (2nd ed.). Wiley-Blackwell. ISBN 978-0-470-67286-0. https://books.google.com/books?id=I59Rasgj3SIC
McGrath 2013, p. 54. - McGrath, Alister E. (2013). Historical Theology: An Introduction to the History of Christian Thought (2nd ed.). Wiley-Blackwell. ISBN 978-0-470-67286-0. https://books.google.com/books?id=I59Rasgj3SIC
McGrath 2013, p. 57. - McGrath, Alister E. (2013). Historical Theology: An Introduction to the History of Christian Thought (2nd ed.). Wiley-Blackwell. ISBN 978-0-470-67286-0. https://books.google.com/books?id=I59Rasgj3SIC
"Sabellianism". Catholic Answers. Retrieved 2021-10-16. In 382 the Council of Rome, with Pope Damasus I presiding, condemned the heresy, 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). https://www.catholic.com/magazine/print-edition/sabellianism
"Sabellianism". Banner of Truth USA. 2016-05-18. Retrieved 2021-10-16. The revelations of Father and Son therefore, to Sabellius, belonged to the past, and the Church now was the Church of the Spirit, and after the end of the age, there would just be God, who would be neither Father, Son, nor Spirit. His teaching was rightly condemned by the Church, which understood that it strikes at the very foundations of Christianity. https://banneroftruth.org/us/resources/articles/2016/sabellianism/
Bernard, David (1993). "Father, Son, and Holy Ghost". The Oneness of God. Word Aflame Press. ISBN 978-0-912315-12-6. Archived from the original on February 9, 2008. The Bible speaks of Father, Son, and Holy Ghost as different manifestations, roles, modes, titles, attributes, relationships to man, or functions of the one God, but it does not refer to Father, Son, and Holy Ghost as three persons, personalities, wills, minds, or Gods. God is the Father of us all and in a unique way the Father of the man Jesus Christ. God manifested himself in flesh in the person of Jesus Christ, called the Son of God. God is also called the Holy Spirit, which emphasizes his activity in the lives and affairs of mankind. God is not limited to these three manifestations; however, in the glorious revelation of the one God, the New Testament does not deviate from the strict monotheism of the Old Testament. Rather, the Bible presents Jesus as the Father, the Son, and the Holy Ghost. Jesus is not just the manifestation of one of three persons in the Godhead, but he is the incarnation of the Father, the Jehovah of the Old Testament. Truly, in Jesus dwells all the fulness of the Godhead bodily. 978-0-912315-12-6
Bernard, David K. (1993). "Jesus is God". The Oneness of God. Word Aflame Press. ISBN 978-0-912315-12-6. Archived from the original on 15 January 2008. Jesus is everything that the Bible describes God to be. He has all the attributes, prerogatives, and characteristics of God Himself. To put it simply, everything that God is Jesus is. Jesus is the one God. There is no better way to sum it all up than to say with the inspired Apostle Paul, "For in him dwelleth all the fulness of the Godhead bodily. And ye are complete in Him" (Colossians 2:9-10). 978-0-912315-12-6
Gill, Kenneth (April 1998). "Dividing Over Oneness". Christian History. Christianity Today. Retrieved 2021-10-16. So went one of the hymns of the Oneness Pentecostals, for whom Jesus was the name of the Father, the Son, and the Holy Spirit. Their desire to recapture the mantle of the apostolic church started with questions over the proper formula to use in water baptism. But they were soon questioning even the doctrine of the Trinity. https://www.christianitytoday.com/history/issues/issue-58/dividing-over-oneness.html
Norris 2009, p. 27. - Norris, David S. (2009). I Am: A Oneness Pentecostal Theology. Hazelwood, Mo.: Word Aflame Press. ISBN 9781567227307. OCLC 312444348. https://search.worldcat.org/oclc/312444348