5 And you know this: that appeared so that those sins may be taken away. And in Him there is no sin: 6everyone who dwells in Him never sins; everyone who sins has not seen Him nor learnt of Him.
The above is my translation. First we here about “that appeared” (the pronoun being ekeinos not autos) which is reminiscent of verse 2, speaking of the future return of Christ, thus identifying Christ as the subject. However what’s interesting is that which was in the future is not being treated as if it were in the past. Both appeared and taken away are in the simple past tense, and even “to see” later in verse 6 which is also reminiscent of verse 2 is in the perfect past tense. So we have this sense that the hope of verse 3 is in some way realized for the believer.
Then we come to the idea of being “in Him” in connection to the idea of sinning. There is no sin in Christ and indeed everyone who is in Him can’t sin. Let us consider verse 6. I believe this represents an elucidation of 5b, shown by the use of the colon above. 6 uses a parallel structure, but not perfectly. We expect him to say “everyone who dwells in Him never sins; everyone who sins never dwells in Him”. This would be simple parallel to say what he is saying. But instead, he switch the dwelling language to seeing and knowing, referring again to verse 2, and this is important. There, John equates seeing Christ as being like Christ.
Now lets put all of this together. See Christ -> become like Christ -> never sin again. Therefore, if one sins -> one is not like Christ -> one has never seen Christ. After all, to be in Christ is to take on Christ’s traits: to be Christ-like.
Therefore, we may know who are in Christ and who are not by the way they live their lives. Those who remain enslaved to sin cannot be truly in Christ because they are nothing like Him! Christ has had victory over sin. Therefore, if you are in Him, so do you.
However, the purpose of this is assurance, not condemnation. We have assurance that what we have hoped for is a present reality in our lives, protecting us from sin. If others try to condemn you, try to say that there is no hope in Christ or that you have no share in the hope of Christ, look at your life. If you are behaving like Christ than be assured that you are in Him, for how us could you come to lifestyle? Be confident and live in the hope that is Christ Jesus.