If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar, and His word is not in us.
– 1 John 1:8-10 NKJV

My little children, these things I write to you, so that you may not sin. And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous. And He Himself is the propitiation for our sins, and not for ours only but also for the whole world.
– 1 John 2:1-2 NKJV

chapter 1

vs 8: We must own the fact that we have sinned, that we are sinners in the need of Salvation. Notice that “have no sin” is past tense.

vs 9: The sin we have committed is fully forgivable through confession (not Roman Catholic Theology) of our sins to God through Jesus the Son. We are then cleansed of those sins. They no longer are charged against us. Notice that “confess our sins” is present tense, the act of forgiveness and cleansing happens here and now, immediately.

vs 10: If we dare to challenge this truth and claim that we have never had sin, then we are both making God to be a liar as well as not having the Word of God in us. The Word of God is also Jesus. Therefore, by not owning that we have (past tense) sinned we are also not allowing Jesus to occupy our lives.

chapter 2

vs 1: My little children, John is addressing those that are of Christ, those that are members of the Body of Christ, Christians.

these things I write to you John writes to the Christians. This is not an address to those who do not know Jesus. This is to those that have understood 1 John 1:8-10 and have confessed their sin, received their forgiveness and cleansing, salvation in Christ.

so that you may not sin. John is letting fellow Christians know that once they have confessed their sin and received forgiveness and cleansing from and of those sins they need not sin again. In 1 John 1:8-10 he speaks of the past into the present tense. Here he is speaking of the future.

And if anyone sins, John acknowledges that some may end up in sin. Note that John does NOT say “when you sin” but “if anyone sins“. Also, note that by using the term “anyone” he is implying not all will find themselves sinning, but only some may find it so.

we have an Advocate with the Father John is saying that even as a Christian, the act of confessing our sins is not a one time process, nor is the effect of cleansing and forgiveness only effective once. He is saying that we have a lawyer that will argue our case before God.

Jesus Christ the righteous. John is identifying that lawyer as Jesus who died for our sins.

vs 2: And He Himself He refers to Jesus just identified as the lawyer.

is the propitiation for our sins Propitiation is an uncommon word that means that Jesus is not only our lawyer but also the payment for our sins. Not only the sins of the past which we confessed but also for the present and future sins we may find ourselves in. But, not that does not mean we must or will purposefully transgress God in the future, only that Jesus is the lawyer and payment for any sins should that occur.

and not for ours only but also for the whole world. The saving grace that is available through Jesus is not reserved only for the Christian, but it is also reserved for all. It is available for all who will, as John points out earlier who confess their sins.