If you do some research you'll find that this trinity "proof text" doesn't really exist and was a later addition. Even the trinitarian commentaries admit it.
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"1 John 5:7-8
7. For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
The text of this verse should read, "Because there are three that bear record". The remainder of the verse is spurious. Not a single manuscript contains the trinitarian addition before the fourteenth century, and the verse is never quoted in the controversies over the Trinity in the first 450 years of the church era.
8. The three witnesses are the spirit, and the water, and the blood: and these three agree in one.
"The trinity of witnesses furnish one testimony" (Plummer, The Epistles, p. 116) namely that Jesus Christ came in the flesh to die for sin that men might live."
(from The Wycliffe Bible Commentary, Electronic Database. Copyright (c) 1962 by Moody Press)
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"1 John 5:7
There are three that bear record] The FATHER, who bears testimony to his Son; the WORD or Logos (NT:3056), who bears testimony to the Father; and the HOLY SPIRIT, which bears testimony to the Father and the Son. And these three are one in essence, and agree in the one testimony, that Jesus came to die for, and give life to, the world. But it is likely this verse is not genuine. It is wanting in every manuscript of this letter written before the invention of printing, one excepted, the Codex Montfortii, in Trinity College, Dublin: the others which omit this verse amount to one hundred and twelve. It is missing in both the Syriac, all the Arabic, AEthiopic, the Coptic, Sahidic, Armenian, Slavonian, etc., in a word, in all the ancient versions but the Vulgate; and even of this version many of the most ancient and correct MSS. have it not. It is wanting also in all the ancient Greek fathers; and in most even of the Latin.
The words, as they exist in all the Greek MSS. with the exception of the Codex Montfortii, are the following:
"6. This is he that came by water and blood, Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth.
7. For there are three that bear witness, the Spirit, the water, and the blood; and these three agree in one.
9. If we receive the witness of man, the witness of God is greater, etc."
The words that are omitted by all the MSS., the above excepted, and all the versions, the Vulgate excepted, are these:
[In heaven, the Father, the Word, and the Holy Spirit, and these three are one, and there are three which bear witness in earth.]
To make the whole clearer, that every reader may see what has been added, I shall set down these verses, with the inserted words in parentheses.
"6. And it is the Spirit that beareth witness, because the Spirit is truth.
7. For there are three that bear record (in heaven, the Father, the Word, and the Holy Spirit, and these three are one.
8. And there are three that bear witness in earth), the Spirit, and the water, and the blood, and these three agree in one.
9. If we receive the witness of men, the witness of God is greater, etc."
(from Adam Clarke's Commentary, Electronic Database. Copyright (c) 1996 by Biblesoft)
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"1 John 5:7
Two or three witnesses were required by law to constitute adequate testimony. The only Greek manuscripts, in any form, which support the words, "in heaven, the Father, the Word, and the Holy Spirit, and these three are one: and there are three that bear witness in earth," are the Montfortianus of Dublin, copied from the modern Latin Vulgate; the Revianus, copied from the Complutensian Polyglot; a manuscript at Naples, with the words added in the margin by a recent hand; Ottobonianus, 298, of the fifteenth century, the Greek of which is a translation of the accompanying Latin. All the old versions omit the words. The oldest manuscripts of the Vulgate omit them, the earliest Vulgate manuscript which has them being Wizanburgensis, 99. of the eighth century. A Scholium quoted in Matthaei shows that the words did not arise from fraud; for all Greek manuscripts ("there are three that bear record"), the Scholiast notices, have "three," masculine, because the three things (the Spirit, the water, and the blood) are SYMBOLS OF THE TRINITY. To this Cyprian, 196, refers: 'Of the Father, Son, and Holy Spirit, it is written, "And these three are one" ' (a unity)."
(from Jamieson, Fausset, and Brown Commentary, Electronic Database. Copyright (c) 1997 by Biblesoft)
Jesus said that the scriptures bear witness to Him and he bears witness to Himself. God the Father bears witness. John 5:
5:36 But I have greater witness than that of John: for the works which the Father hath given me to finish , the same works that I do, bear witness of me, that the Father hath sent me.
5:37 And the Father himself, which hath sent me, hath borne witness of me. Ye have neither heard his voice at any time, nor seen his shape.