1John 1:9 - If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.

Sin is what separates us from God and keeps us out of Heaven. However, when we confess our sins (ie: agree with God that it IS sin), God will forgive us. The blood of Christ, shed for our sins, cleanses us and makes us fit for Heaven.