Biblical Unitarians, along with virtually all modern biblical scholars and textual critics, affirm that the Comma Johanneum in 1 John 5:7-8 ('For there are three that testify in heaven: the Father, the Word, and the Holy Spirit; and these three are one') is not part of the original text of 1 John. This passage is absent from all early Greek manuscripts, the earliest versions, and most Old Latin manuscripts. It appears first in the Latin Vulgate of Jerome (late 4th century) and was gradually incorporated into later Greek texts.

The phrase is widely recognized as a scribal interpolation that conflates 1 John 5:8 (three earthly witnesses) with a Trinitarian gloss. Church fathers like Origen, Tertullian, and Augustine were unaware of this reading. The verse's vocabulary and syntax reflect post-Nicene theological developments rather than first-century epistolary style. For Biblical Unitarians, the absence of this verse from the original text strengthens their case that Trinitarian doctrine is a later theological development rather than taught explicitly in the New Testament.