"He is the propitiation for our sins: and not for ours only, but also
for the sins of the whole world." Does Hell exist?2:2
They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us."
Many Christians interpret this verse to mean that it is impossible for a true believer to become an unbeliever. (They become
antichrists instead.) 2:19
"There are three that bear record in heaven, the Father, the Word, and the Holy Ghost."
This is the clearest, and pretty much the only, expression of the Trinitarian concept in
the Bible. It is not, however, found in the earliest Greek manuscripts and is omitted from most
modern translations. Here, for example, are verses 5:7-8 in the New Revised
"There are three that testify: the
Spirit and the water and the blood, and these three agree."
Not all Christians agree about how these verses (often called
the Johannine comma) should be treated. See here for a KJV-only defense.