Well, the whole "We sin in our flesh but yet we don't sin spiritually because our sin is not charged to our account because of Jesus" type belief is not taught in the Scriptures. For when you quote 1 John 3:9: You also have to look at the surrounding text or verses in the same epistle. 1 John 2:1 says we are not to sin, but if we do sin, we have an advocate named Jesus Christ that we can go to. Why go to Jesus if you are already forgiven in him? 1 John 3:7 says, "Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous." Then 1 John 3:8 says, "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." So we know according to the context right here that he that sins is of the devil. How do we know for sure? 1 John 3:10 confirms it. It says, "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." In other words, 1 John 3:6, 9 are talking about how the true born again believer does not live in habitual sin as a way of life. This is because they are changed spiritually.
As for 1 John 1:8-9 in regards to salvation: Well, 1 John 1:6-7 says, "If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth: But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin." In other words, we see a contrast here. If one walks in darkness (or sin), and says they have fellowship with Him (Jesus), they lie and do not the truth. But if they walk in the light (as he is in the light), then the blood cleanses them of all sin. But you believe all your sins are cleansed by a belief alone type Theology, though. Yet, 1 John 1:7 says otherwise. You have to walk in the Light as He is in the Light. "Walking in the Light" is in reference to walking in His commands. For the next chapter makes this point clear (See 1 John 2:3-6). Besides, 1 John 1:9 says... IF we confess our sins, he is faithful and just to FORGIVE us our sins. So it goes to reason that if you are forgiven by confessing your sins, then not doing so would mean you not be forgiven. This fact is confirmed for us in the Parable of the Tax Collector and the Pharisee (Luke 18:9-14). The Tax Collector was more justified than the Pharisee because he cried out to God to have mercy on him being a sinner. Yet, the Pharisee was proud and thought he was just God's child and did not need to confess or repent (Like many OSAS proponents), and he thought he was better than the Tax Collector.