More evidence that the King James Bible is correct with its inclusion of texts like 1 John 5:7:
Jerome noted that unfaithful translators were removing 1 John 5:7 which is apparently why it didn’t make it into the majority of Greek manuscripts we have today.
Jerome stated this in Prologue to the Canonical Epistles:
“Just as these are properly understood and so translated faithfully by interpreters into Latin without leaving ambiguity for the readers nor [allowing] the variety of genres to conflict, especially in that text where we read the unity of the trinity is placed in the first letter of John, where much error has occurred at the hands of unfaithful translators contrary to the truth of faith, who have kept just the three words water, blood and spirit in this edition omitting mention of Father, Word and Spirit in which especially the catholic faith is strengthened and the unity of substance of Father, Son and Holy Spirit is attested.”
Additionally, Cyprian, writing earlier than Jerome quotes it:
“He who breaks the peace and the concord of Christ, does so in opposition to Christ; he who gathereth elsewhere than in the Church, scatters the Church of Christ. The Lord says, ‘I and the Father are one;’ and again it is written of the Father, and of the Son, and of the Holy Spirit, ‘And these three are one.'”
-Cyprian (A.D. 250), On the Unity of the Catholic Church, Ch. 6
To just say that 1 John 5:7 isn’t inspired because it isn’t in Sinaticus and Vaticanus isn’t telling the truth. Several church fathers prior to the 4th century also quote it. Much much more is said about it here.