There are three different versions of this particular verse in the manuscripts.
The majority of manuscripts, as well as the Greek text that is used in the New Testament Greek churches today, read:
Θεὸς ἐφανερώθη ἐν σαρκί,
ἐδικαιώθη ἐν Πνεύματι,
ὤφθη γγέλοις,
ἐκηρύχθη ἐν ἔθνεσιν,
ἐπιστεύθη ἐν κόσμῳ,
νελήφθη ἐν δόξῃ.
God was revealed in the flesh,
vindicated in [the] spirit, seen by angels,
preached among the nations,
believed on in the world,
and received up in glory.
(EOB:NT)
In the Sinaiticus (4th c.), Alexandrinus (5th c.), Ephraimi (5th c.), Augiensis (9th c.), and Boernerianus (9th c.) codices, the relative pronoun ὅς takes the place of Θεὸς. The NASB editors and (presumably) others have chosen this particular reading over the majority reading.
In his Textual Commentary on the Greek New Testament (2nd ed.), Bruce Metzger explains the presence of Θεὸς in some manuscripts as being due to either transcription error or deliberately for stylistic reasons:
Thus, no uncial (in the first hand) earlier than the eighth or ninth century (Ψ) [Athous Lavrensis] supports θεός; all ancient versions presuppose ὅς or ὅ; and no patristic writer prior to the last third of the fourth century testifies to the reading θεός. The reading θεός arose either (a) accidentally, through the misreading of OΣ [the pronoun] as ΘΣ [abbreviation for θεός], or (b) deliberately, either to supply a substantive for the following six verbs, or, with less probability, to provide greater dogmatic precision.
p. 574
Among the Greek Church Fathers who cited this particular passage are Gregory of Nyssa (335-394 AD), John Chyrsostom (347-407 AD). The oldest Codex which Metzger cites is the Sinaiticus, which scholars date to between 330 and 360 AD. Thus, it is arguable which of the witnesses is more historical.
In any case, I do not think it is terribly significant whether the passage indicates ὅς or θεός. The above cited writings of the Church Fathers (as well as others, such as Theodoret of Cyrus), show that regardless of what some texts may have indicated, they clearly understood the passage to refer to God.
Brother David doesn’t bother with research or reason. So not only do the earliest surviving manuscripts (which Stewart rejects as “Satanic”) have the relative pronoun “ὅς”, also the indirect evidence of the earliest writers who quoted this passage and wrote “ὅς” is pretty clear. Historians usually choose what is called the “lectio difficilior” (the “more difficult version”), because ancient texts invariably change from a difficult to an easier (or clearer) version when being copied by scribes, often because the scribe isn’t sure about the original he’s copying and substitutes an easier reading.