According to this verse, sin separates the believer from fellowship with God. This is also why 1 John 1:9 is such a vital and daily practice. There are no degrees to sin, according to the Bible. Perhaps there is a habitual problem of using the Lord's name in vain, maybe a lifestyle living in the sin of homosexuality, or even constant worrying (worry is a sin - Phil. 4:6), all sin needs to be confessed, but more importantly, all sin separate believers from fellowship with God.
Isaiah 59:2 "But your iniquities have separated between you and your God, and your sins have hid his face from you, that he will not hear."
Not just fellowship, but also prayer. When we pray with unconfessed sin in our hearts and lives, the Lord does not hear them. There are a lot of times that people tell me that they are praying for this or that and yet, I know, they have unconfessed sin in their lives. Most often, this seems to be prevalent in those who are decidedly living in a lifestyle opposed to Biblical values. All Christians sin, but the Holy Spirit convicts. Living in a blatant and rebellious lifestyle is shaking your fist to God, yet Christians everywhere are living in unconfessed sin.
1 John 1:9 "If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness."