1 John 5:7 (KJV)
7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
This verse teaches that the Father, the Son, and the Holy Spirit are one God.
The overwhelming majority of qualified experts in textual criticism believe that the words after "bear record" in this verse, as rendered above in the KJV, are late additions into the text. Many modern Bibles render this entire verse as something like, "There are three that testify".
The Trinitarian Wayne Grudem explains:
Wayne Grudem - Systematic Theology, Chapter 14
However, the KJV translation of 1 John 5:7 should not be used in this connection. It reads, "For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one." The problem with this translation is that it is based on a very small number of unreliable Greek manuscripts, the earliest of which comes from the fourteenth century AD. No modern translation (except the NKJV) includes this KJV reading, but all omit it, as do the vast majority of Greek manuscripts from all major text traditions, including several very reliable manuscripts from the fourth and fifth century AD, and also including quotations by church fathers such as Irenaeus (d. ca. AD 202), Clement of Alexandria (d. ca. AD 212), Tertullian (d. after AD 220), and the great defender of the Trinity, Athanasius (d. AD 373).
Therefore, the part of the verse which is supposed to teach Trinitarianism is not original to the Bible. It is a rather late addition, which many think worked its way into the text from Latin translations.
If it were genuine, it would say about as much as John 10:30 (covered here), except it would add that the Holy Spirit is in agreement with the Father and Son, which Unitarians would acknowledge anyway. But, it is not genuine.