This chapter speaks of truth, lies, light and darkness and the Antichrist. If I recall correctly this is the only place that term is used or defined in the Bible.
1Jn 2:3 And hereby we do know that we know him, if we keep his commandments.
1Jn 2:4 He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.
1Jn 2:21 I have not written unto you because ye know not the truth, but because ye know it, and that no lie is of the truth.
1Jn 2:22 Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son.
1Jn 2:23 Whosoever denieth the Son, the same hath not the Father: (but) he that acknowledgeth the Son hath the Father also.
1Jn 2:18 Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time.
1Jn 2:19 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: but they went out, that they might be made manifest that they were not all of us.
1Jn 2:26 These things have I written unto you concerning them that seduce you.
1Jn 2:25 And this is the promise that he hath promised us, even eternal life.
1Jn 2:28 And now, little children, abide in him; that, when he shall appear, we may have confidence, and not be ashamed before him at his coming.
1Jn 2:29 If ye know that he is righteous, ye know that every one that doeth righteousness is born of him.
See Old John Gill and Albert Barnes for a good discussion on antichrist(s).
Here is the beginning of Barnes’ comment on verse 18:
And as ye have heard that antichrist shall come – The word “antichrist” occurs in the New Testament only in these Epistles of John, 1Jn 2:18, 1Jn 2:22; 1Jn 4:3; 2Jn 1:7. The proper meaning of (ἀντί anti) in composition is:
(1) “Over-against,” as ἀντιτάσσειν antitassein;
(2) “Contrary to,” as ἀντιλέγειν antilegein;
(3) Reciprocity, as ἀνταποδίδωμι antapodidōmi;
(4) “Substitution,” as ἀντιβασιλεύς antibasileus;
(5) The place of the king, or ἀνθύπατος anthupatos – “proconsul.”
The word “antichrist,” therefore, might denote anyone who either was or claimed to be in the place of Christ, or one who, for any cause, was in opposition to him.
Read more on your own.
But here is a message we can all understand.
1Jn 2:7 Brethren, I write no new commandment unto you, but an old commandment which ye had from the beginning. The old commandment is the word which ye have heard from the beginning.
1Jn 2:8 Again, a new commandment I write unto you, which thing is true in him and in you: because the darkness is past, and the true light now shineth.
1Jn 2:9 He that saith he is in the light, and hateth his brother, is in darkness even until now.
1Jn 2:10 He that loveth his brother abideth in the light, and there is none occasion of stumbling in him.
1Jn 2:11 But he that hateth his brother is in darkness, and walketh in darkness, and knoweth not whither he goeth, because that darkness hath blinded his eyes.
A few days ago I posted a short comment on light.