The author, John, was a Jew, but he did not become a child of God until after he came to faith in Jesus. (See: rc://en/tw/dict/bible/kt/faith)
Cain was a son of the first man, Adam, and the first woman, Eve. He murdered his brother out of jealousy or envy. This reference is difficult to understand if Genesis has not yet been translated. Additional explanation may be necessary.
The verb “to know” is used in a way that means “to understand.” It is a type of knowledge based on one’s experiences. Care should be taken when translating this word because it is not the primary definition of the term.
There are many instructions in this chapter that will change their meaning if they are translated as happening continually or in another way. This passage is especially difficult in light of the fact that the first chapter teaches that Christians inevitably sin. (See: rc://en/tw/dict/bible/kt/sin)
Many scholars believe this is about remaining in the will of God and is not a reference to a person’s eternal salvation. (See: rc://en/tw/dict/bible/kt/eternity and rc://en/tw/dict/bible/kt/save)