The short answer: No; it seems that partialism is not a "real," historically defined heresy.
Explanation: Before writing this post, I checked the applicable titles from among my usual textual sources -- a variety of historic theological works that are now in the public domain and available online. When that yielded no references to "partialism," I went several pages deep on various Google searches. That search turned up many pages that described "partialism," although few of them listed any references, and those that did and those that did not all seemed to have come from one of two sources: Either a page at Monergism.com on Trinitarian Heresies, or a YouTube video entitled "St. Patrick's Bad Analogies." I then wrote the answer that follows, which offers some historic information on the doctrine of the Trinity, but does not reference "partialism" directly.
Some time later, I decided to continue looking for answers by visiting a seminary here in town that has a pretty heavy duty library. Having searched there extensively, I'm now going to go ahead and say: "Partialism" is not an historically defined Trinitarian heresy.
My search at the seminary library included extensive online databases of academic theological journals; no less than twenty separate dictionaries of Christianity that covered the entirety of church history in (tens of?) thousands of pages; multiple encyclopedias of Christianity; and various other books. There was not a single, solitary occurrence of the word "partialism" to be found anywhere.
The reason I mentioned all this is to illustrate how it is that I reached the bolded conclusion above. While I of course could be wrong, it's also interesting to note that, in the "St. Patrick's Bad Analogies" video, where historic references are offered for various of the Trinitarian Heresies that are described, when "Partialism" is mentioned, a joke is offered in place of any history.
Perhaps "partialism," with its many internet references, belongs on Snopes.com?
The Short Answer: The Council of Nicea is one historic event that, though it did not addresses Partialism by that name, did clarify the orthodox doctrines regarding the ideas expressed in Partialism.
This answer describes some church history events from the fourth century CE in detail. It specifically looks at the Council of Nicea, the Arian Controversy, and the work of Athanasius, all of which directly addressed the ways in which the Father and Son relate to each other as two Persons of the Trinity.
It does not directly address partialism by that name, however, and is offered as relevant historical background, as the Council of Nicea arguably created the frame within which all following Trinitarian doctrine was developed. (It's also a nice opportunity for me to make a few notes to summarize some of my recent reading).
The Council of Nicea and the Arian Controversy1
The Council of Nicea, or First Ecumenical Council, was held in 325 CE. The Roman Emperor Constantine had called the meeting of bishops from across his newly unified empire to decide the orthodox stance on various doctrinal issues, and especially to resolve the Arian Controversy.
The Arian Controversy had begun a few years earlier, as a dispute between two leaders of the Christian Church in Alexandria: Bishop Alexander of Alexandria, and a popular presbyter named Arius. The main issue was whether or not the Son was co-eternal with the Father, or whether the Son was created by the Father. The latter was Arius' teaching. While it may sound like a theological subtlety, it actually had profound soteriological implications at a point in history when many of the doctrines that we take for granted today had not yet been decided.
When the time came for the presentation of the Arian perspectives at the Council, Bishop Eusebius of Nicomedia -- not to be confused with Eusebius of Caesaria, the widely regarded "Father of Church History" -- presented the ideas taught by Arius', with whom he agreed, in a speech, since only bishops could address the Council. The response of the majority of the bishops in attendance was condemnatory, and some went so far as to tear Eusebius' notes from his hands and trample them underfoot, shouting "blasphemer!" and "heretic!"
The Nicene Creed was formulated by the Council as a response to Arianism. Emperor Constantine himself -- perhaps prompted by an advisor -- suggested the use of the Greek word homoousios, "of the same substance," to describe the nature of the Father and Son, and this word was included in the Creed. The official response to the Arian Controversy, now the Heresy of Arianism, seemed settled.
The Resurgence of Arianism and the Response Led by Athanasius of Alexandria2
Not everyone accepted the findings of the Council of Nicea, however, and some of those who continued to embrance Arianism had significant political influence. The state had a important role in determining what was and what was not acceptable church teaching, and the matter was further complicated when Constantine died and was succeeded by his three sons, each of whom ruled over a different portion of the Roman Empire. Constantius, the son who ruled over the eastern portion of the empire, including Alexandria in Egypt, was pro-Arian, and years later would come to be the sole emperor in 353 CE.
Enter Athanasius of Alexandria, who had attended the Council of Nicea as a young man, where he was part of Alexander's group. Athanasius was not known as a particularly intellectual man, though he did write extensively, and neither were his political connections great, but he was widely revered for his monastic discipline and holiness of life. In 328 CE he became bishop of Alexandria, though he did not seek the office and even tried to avoid being appointed. He would go on to lead the response against the Arian resurgence.
As a result of his efforts, Athanasius endured suffering both from the state and from other leaders of the church. He was banished from and returned to Alexandria on more than one occassion, and his opponents spread rumors to the effect that he practiced occult magic, that he was a tyrant over the Alexandrian church, and even that he was a murderer -- all of which rumors he successfully defeated in time. Athanasius remained "a man of the people," and was given a hero's welcome when he returned from the banishment imposed by state officials, fearful as they were of his ability to influence the public.
After several years of many individuals working to teach the Nicene faith, Arianism was on the decline, with the majority of the church rejecting the idea that the Son was created by the Father. Many of those who plainly rejected Arianism were reluctant to accept the language of the Nicene Creed however, due to a concern that the word homoousios could be interpreted as meaning that there was no distinction between the Son and the Father (which was related to Patripassionism, another doctrinal controversy). The word homoiousios, "of a similar substance," was suggested as a more accurate description of the relation of the Father and the Son.
Initially, Athanasius said that those who favored the term homoiousios were just as heretical as the Arians. He stated his doctrine in no uncertain terms in his Four Discourses Against the Arians:3
As we said above, so now we repeat, that the divine generation must not be compared to the nature of men, nor the Son considered to be part of God, nor the generation to imply any passion whatever; God is not as man; for men beget passibly, having a transitive nature, which waits for periods by reason of its weakness. But with God this cannot be; for He is not composed of parts, but being impassible and simple, He is impassibly and indivisibly Father of the Son.
Continued study and debate over the next few years led him to a more comprehensive view, however. His final determination on the matter, as summarized by church historian Justo Gonzalez, was as follows:4
Finally, in a synod gathered in Alexandria in 362 CE, Athanasius and his followers declared that it was acceptable to refer to the Father, Son, and Holy Spirit as "of one substance" as long as this was not understood as obliterating the distinction among the three, and that it was also legitimate to speak of "three substances" as long as this was not understood as if there were three gods.
A few years after Athanasius' death, the Second Ecumenical Council was held in Constantinople in 381 CE, and the doctrine of the Council of Nicea was ratified. In addition to establishing the equality of the Holy Spirit within the Trinity, the Council further established the Nicene understanding as orthodox teaching across the Empire, and again condemned the Arian teaching as heretical.
The emphasis during this period of history on the particular words used to describe the relative equality of the "substance" of which the Father and Son are composed seems to speak directly to the concept of Partialism.
- The Story of Christianity, Revised Edition, by Justo Gonzalez. Chapter 17, "The Arian Controversy and the Council of Nicea."
- Gonzalez. Chapter 19, "Athanasius of Alexandria."
- Four Discourses Against the Arians. Discourse One, paragraph 28. New Advent Catholic Encyclopedia.
- Gonzalez. Chapter 17.