The following is from the Institute for Creation Research:
For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. (1 John 5:7)
The above text is the clearest and most succinct statement of the doctrine of the Trinity in the Bible, but also probably the most controversial verse in the Bible. Many authorities believe it was not in John's original epistle, but was added by some ancient copyist. Consequently, although it is in the King James Bible, most modern English translations omit it. There are others (including this writer) who are convinced it was in the original and should be retained.
In any case, the truth of the Trinity is taught in many other passages of the New Testament and does not depend on this particular verse. It is undoubtedly the most distinctive doctrine of the Christian faith. Most religions are either pantheistic (e.g., Buddhism) or polytheistic (e.g., Hinduism). Two other religions are monotheistic (Judaism, Islam), but only Christianity recognizes the one real God (the Triune God--Father, Son, Holy Spirit)--one God in three Persons.
The sacred book of the Muslims--the Koran--regards Jesus as only a prophet, repeatedly denouncing as infidels all those who believe in the Trinity. The Jews often consider Jesus to have been a great teacher, but no more than that. To the Christian, however, the Lord Jesus Christ is a person yet God incarnate, God's only begotten Son--Creator, Savior, King of kings and Lord of lords. Christians also believe that the Holy Spirit is not just a spiritual influence, but a real person, the third Person of the Godhead.
Read more: https://www.icr.org/article/2494/
The Only True God by Henry Morris (ICR) http://www.icr.org/article/9783
Three in One in Genesis by Paul Taylor (Creation Moments) http://www.creationmoments.com/radio/transcripts/three-one-genesis
Our Triune God by John MacArthur (GTY) https://www.gty.org/library/articles/A215/our-triune-god