"If we confess our sins, He is faithful and righteous 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 1-9
In context the promise of that verse is on the condition that we are "walking in the light" (verse 6-7). When we walk in the light, we obey God in everything that we know. Therefore, it is referring to sins of ignorance when the Holy Spirit exposes them.
A born-again Christian does not COMMIT willful sin:
1 John 3:9 Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
If a person COMMITS sin, they are a child of the devil:
1 John 3:8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
1 John 3:10 In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.