Baptism in (Biblical) Unitarian denominations

Pardon while I go off topic for a second, but I do need to address a comment you made before I actually address the question. This is not intended as a criticism towards you, but is really intended to assist the average reader so they can understand the issue better.

When you refer to Christadelphians as a “Unitarian denomination,” while such a statement may be accurate for all intents and purposes, it is not quite specific enough. For example, there are differences between the quote-unquote, “Biblical Unitarian” and the standard run-of-the-mill “Unitarian.” The term “Unitarian” is more of a qualitative term. It describes a certain type or essence of theology, namely, that God is one sole individual. The sole “essence” of their theology is simply that God is one sole individual. Where it gets more complex, is just “who” is that one individual? For the “Biblical Unitarian,” it's “the Father.” For the Christadelphian, it's “the Father.” For the Jehovah's Witness, it's “the Father.” However, there are other groups that fall under the “Unitarian” umbrella that suggest that one individual is the pre-incarnate Christ, who existed as “the Father.” So to use the phrase, “Unitarian denomination,” it doesn't seem quite specific enough. And depending on which “Unitarian” group you are referring to can impact the understanding of Matt. 28:19. The differences among Unitarian types really comes down, not so much to Theology, but to Christology. They all agree with one another that God is one sole individual. But the “Biblical Unitarian” (i.e., Socinian) believes in what's referred to as a, “human Jesus” christology. Oneness Pentecostals (depending on it's denominational affiliation) may hold to some form of Modalism (i.e., Jesus is the Father), though with some variation. Still, Jehovah's Witnesses believe Christ pre-existed with the Father. Yet, they all hold to a “Unitarian” theology. And sometimes they will impose their “Unitarian” framework (and worldview) onto other theologies that are in fact, not Unitarian. The point I'm making: Just recognize the difference between the varying Unitarian groups, because it can have a bearing on your question. While all Biblical Unitarian's are Unitarian, not all Unitarians are “Biblical Unitarian.” Some people do not recognize the distinction (though you may) and tend to conflate the two, so I needed to point that out.

You asked specifically whether “Biblical Unitarian” churches follow the baptismal formula in Matt. 28:19. And to that, I don't suppose all Biblical Unitarians hold the same view regarding the baptismal formula as it is found in Matt. 28:19. But more often than not, Biblical Unitarians (particularlly those that are not familiar with the science of textual criticism) will suggest the words as they are found in Matt. 28:19 are not genuine. Again, not all Biblical Unitarians do this, but those of which you may run across on the web, almost certainly!

When it comes to Matt. 28:19, the most common among Unitarian “trends” is to latch onto a hypothesis made by Frederick C. Conybeare in the earlier half of the 20th century. Conybeare had documented multiple instances in the 4th c. church historian, Eusebius of Caesarea's written works, particularly in the instances where he had alluded to or cited Matt. 28:19 (or, at least in part),

πορευθέντες μαθητεύσατε πάντα τὰ ἔθνη ἐν τῷ ὀνόματί μου
(“Go and make disciples of all nations in My name”)^[1]

Conybeare speculated that because Eusebius was a citizen of (or occupied a post in) Caesarea, that perhaps his version of the account should be given notable weight. In prior generations, the library in Caesarea (under Origen's custodianship) had a vastly expanded library; therefore, it was thought that Eusebius had access to more (and probably better quality) manuscripts which pre-dated all others, especially those being used in the translations of Conybeare's day.

Keep in mind, in 1902 textual critics of the era weren't necessarily working with a copious amount of data. Most translations of the era relied on a rather limited number of mss. It was just some sixty years earlier in 1844 that א (Codex Sinaiticus) was first unveiled by Tischendorf. And even then there were no scholarly publications that took into account the newly found manuscript until 1881, which received considerable amount of push back (as most new publications do when they go against the concensus of the time). Because of this, most textual critics of the day were perhaps quite hesitant to use א as apart of their arsenal. One of the very first text critical apparatus' to even consider א was the critical apparatus known as the Westcott-Hort Greek NT, first published in 1881. Further, in the years (1906 to be exact) following Conybeare's written work on Matt. 28:19, the text critic community was once more blessed with a new discovery: Codex Washingtonianus (a codice dated to the 5th c., and is representative of both, the Western and the Byzantine traditions).

Some fifty years would then go onto pass before something rather significant would, yet again, bless the text critical community: The discovery of the Bodmer papyri, which consists of (22) papyri dating back to the 2nd, 3rd, 4th, and 5th centuries. And since then, over the years even more mss have been unveiled. Whereas in 1902, the contents of א were still being scrutinized and debated among the scholarly community, text critics still had only very limited data to work with. With the discovery of these earlier papyri, it gave text critics more data to work with (and that, from earlier periods), and a bigger “field” to play in. You can't play football in a two hundred square foot space, if you get the gist.

With this “new” data unveiled, this now opened the doors for text critics to study manuscript and textual traditions more fluidly. They were able to study the (sometimes quite distant) relationship between a set of manuscripts. In the first eight chapters of the Gospel of John, for example, א follows a Western tradition. But in Vaticanus those same eight chapters follow an Alexandrian tradition. And even though they follow different textual traditions within those first eight chapters, at crucial points, the two texts agree. What this points to is a common ancestor (though, as previously, it could be quite distant), whether it be a sister, a cousin, a brother from another mother – surely, you understand. And some of these earlier papyri that were discovered only help to validate this. When two manuscripts from completely different regions of the world follow the same textual tradition (keeping the period from which each mss originated in mind), what could that possibly indicate?

That means for the reading to have survived, it had to have crossed any number of lands (being recoppied each time), and probably over the period of a hundred or more years, before it eventually landed in the lap of a scribe who would then include it in their copy. And that's where Conybeare's earlier suggestion to give Eusebius' account more weight because he believed Eusebius had access to earlier data falls short (not to mention those shortcomings pointed out in Bernard Henry Cuneo’s, The Lord’s Command to Baptise: An Historico-Critical Investigation With Special Reference to the Works of Eusebius of Caesarea). Because already by the 5th c. the formula (as it is found in Matt. 28:19), was already so well attested throughout the Christian world, existing in Greek witnesses (and independent authors)^[2] from the Byzantine tradition (A, C, W), the Alexandrian tradition (א, B), the Western tradition (D), and every early versional witness from the 4th-5th c. period (Coptic, Syriac, Georgian, Latin, the list goes on) which indicates that those scribes, in each their own language, were working from a Greek source that included the formula. For the reading to be so prominent by the 5th c. leads one to ask just how early such reading had to have been in order to sweep/flourish the Christian world so early?

For Conybeare's hypothesis to even work, shouldn't there be at least one Greek, Latin, Coptic, or Syriac mss that reads the way he suggests? Not to mention, Eusebius also cites Matt. 28:19 using the longer form, and in a work he also uses the shorter form.^[3]

The standard Unitarian rhetoric regarding Matt. 28:19 is so far detatched from reality. They presuppose the arguments put forth by Conybeare has yet to be refuted, when in 1923, Bernard Henry Cuneo did just that in his work entitled, The Lord’s Command to Baptise: An Historico-Critical Investigation With Special Reference to the Works of Eusebius of Caesarea.

[1] Eusebius, Eccleciastical History, III:6:9
[2] Irenaeus, Against Heresies, III:17; Tertullian, De Baptismo, Chapter 13
[3] Eusebius, Syriac Theophania, IV:8