In I John 2:1 the apostle John is writing to Christians in the world and he addresses them with tenderness as he calls them my little children or born ones.  John continues; These things write I unto you, that ye sin not.  And if any man sin, we have an advocate with the Father, Jesus Christ the righteous:  

Believer’s are commanded not to sin because sin breaks the fellowship we have with God.  If we do sin, we have an advocate with God the Father, Jesus Christ the righteous.

An advocate is he who is called to one’s side or to one’s aid.  In a court of justice, an advocate is the counsel for one’s defense.  He pleads another’s cause or acts as an intercessor before the judge. When a Christian sins, (and all sins are against God Psalm 51:4), Jesus Christ, God’s Son pleads the believer’s case and intercedes with God the Father on our behalf.

When believers access the throne of grace by faith in prayer, Christ intercedes for us.    Seeing then that we have a great high priest, that is passed into the heavens, Jesus Christ the Son of God, let us hold fast our profession. For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.  Let us therefore come boldly unto the throne of grace, that we may obtain mercy (only given to the guilty) and find grace (divine favor) to help in time of need.  Hebrews 4:14-16

Jesus Christ, the believer’s substitute, died in our place.  He died for all our sins….past, present and future.  As our defense counsel (advocate), having paid our sin penalty Himself, He intercedes for us as we confess our sins, declaring that the believer’s sins are paid for in full by his own sacrifice.  By the which will we are sanctified (set apart) through the offering of the body of Jesus Christ once for all.  And every priest standeth daily ministering and offering oftentimes the same sacrifices, which can never take away sins:  But this man, after he had offered one sacrifice for sins forever, sat down at the right hand of God; from henceforth expecting till his enemies be made his footstool. For by one offering he hath perfected forever them that are sanctified (set apart). Hebrews 10:10-14  

When God the Father looks at believers, He does not see our sin. He sees only the righteousness of His Son.  God’s justice is satisfied, on the behalf of believers, in the substitutionary death of Jesus  Christ.

The Apostle John gives specific instructions on what to do when a believer does sin.  If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness I John 1:9.  To confess is to speak the same thing or to agree with.  It is to admit one’s guilt as a result of inward conviction.  

When we confess our sins to God, we are saying the same thing about them that He says.  Confession which results in cleansing occurs when believers agree with God against ourselves.

The Scriptures describe the life that a believer receives from God as eternal.  Eternal life begins when one accepts God’s provision for his sin.  Eternal life is life that never ends.   To commit an act of sin does not change the type of life received from God when trusting Christ as one’s only hope of salvation.

What is affected by a believer’s sin is his fellowship with the Father and his inward peace and joy.  We must keep short accounts with God.  The believer’s goal should be to have no unconfessed sin.

When we recognize sin in our life or in our heart, we must deal with it immediately by confessing it to God.  By faith we consider it to be cleansed and forgiven according to I John 1:9.  

To strengthen oneself against the power of sin, believers should obey Psalm 119:11.  Thy word have I hid in my heart, that I might not sin against thee (God).