Textual Note - 1 John 5:7-8

The doctrine of the Trinity is clearly taught from other texts throughout the Bible. However, 1 John 5:7 is probably a later insertion. The earliest Greek manuscripts of 1 John that we have do NOT contain the bulk of this verse and some of verse 8.

Consider this commentary by Bruce Metzger:

External Evidence. (1) The passage is absent from every known Greek manuscript except eight, and these contain the passage in what appears to be a translation from a late recension of the Latin Vulgate. Four of the eight manuscripts contain the passage as a variant reading written in the margin as a later addition to the manuscript.

(2) The passage is quoted by none of the Greek Fathers, who, had they known it, would most certainly have employed it in the Trinitarian controversies (Sabellian and Arian). Its first appearance in Greek is in a Greek version of the (Latin) Acts of the Lateran Council in 1215.

(3) The passage is absent from the manuscripts of all ancient versions (Syriac, Coptic, Armenian, Ethiopic, Arabic, Slavonic), except the Latin; and it is not found (a) in the Old Latin in its early form (Tertullian Cyprian Augustine), or in the Vulgate (b) as issued by Jerome (codex Fuldensis [copied A.D. 541-46] and codex Amiatinus [copied before A.D. 716]) or (c) as revised by Alcuin (first hand of codex Vallicellianus [ninth century]).

B) INTERNAL PROBABILITIES. (1) As regards transcriptional probability, if the passage were original, no good reason can be found to account for its omission, either accidentally or intentionally, by copyists of hundreds of Greek manuscripts, and by translators of ancient versions.

(2) As regards intrinsic probability, the passage makes an awkward break in the sense.

For the story of how the spurious words came to be included in the Textus Receptus, see any critical commentary on 1 John, or Metzger, The Text of the New Testament, pp. 101 f.; cf. also Ezra Abbot, "I. John v. 7 and Luther's German Bible," in The Authorship of the Fourth Gospel and Other Critical Essays (Boston, 1888), pp. 458-463.

For more regarding the Trinity see:

What is the Trinity?
The Trinity or the Truth?
Does the Doctrine of the Trinity Matter?

Reference

Bruce Manning Metzger and United Bible Societies, A Textual Commentary on the Greek New Testament, Second Edition a Companion Volume to the United Bible Societies' Greek New Testament (4th Rev. Ed.), 647 (London; New York: United Bible Societies, 1994).