In this chapter John writes about both a specific antichrist and many antichrists. The word "antichrist" means "opposed to Christ." The antichrist is a person who will come in the last days and imitate Jesus' work, but he will do it for evil. Before this person comes, there will be many people who work against Christ; they too are called "antichrists." (See: [[rc://en/tw/dict/bible/kt/antichrist]] and [[rc://en/tw/dict/bible/kt/lastday]] and [[rc://en/tw/dict/bible/kt/evil]])
Being in God is a metaphor for having fellowship with God, and God's word and truth being in people is a metaphor for people knowing and obeying God's word.
Walking is a metaphor for behaving, not knowing where one is going is a metaphor for not knowing how to behave, and stumbling is a metaphor for sinning.
The light is a metaphor for knowing and doing what is right, and darkness and blindness are metaphors for not knowing what is right and doing what is wrong.