The major sins for an unbeliever are unbelief, selfishness, rebellion, godlessness.
I am still having trouble understanding your view that Christians cannot sin. We all know godly leaders who are genuine believers who have had affairs. The moral law of God embodied in the 10 commandments calls adultery sin. God judges the sin of the unbeliever, but He also judged sin in the camp of His people Israel or the Church. A holy God does not call it sin if committed by an unbeliever and the same act not sin if committed by a believer. Many falllen believers have been restored after repentance and renewed obedience. I do not think this takes away from the fact we are in Christ, have His life/righteousness, etc. Repentance seems to be a concept applied initially when we turn from Self to the Savior and then in another sense if we lapse as a believer (sinful motive, act, thought, etc.). I John, Cor., Hebrews etc. seems to teach that believers do not have to sin, should not sin, but that IF they do sin, we have an advocate (I Jn. 1:9 If we confess...He is faithful...I would note the continuous present tenses that distinguish an ongoing lifestyle of sin as an unbeliever from an isolated sin as a believer). I understand that you feel that a believer cannot possibly sin or that it is automatically not counted against them, but I do not think this is explicit in Scripture and would leave a believer without genuine free moral agency. Yes, we have the mind of Christ and our wills should conform to His will, but numerous exhortations seem to indicate it is possible to quench and grieve the Spirit because we have self-determining wills pre and post conversion. We are even more free as a believer and are not in the bondage of sin. This still does not necessitate that we cannot possibly commit adultery, for example (which surely is a sin, no matter how you slice it).
I have never committed adultery, but if I did, I would expect the conviction of the Spirit, I would repent, ask forgiveness of God and man, and go and sin no more. I would be deceiving myself with lies if I thought God did not see my acts because of His blood and that I can persist in adultery with impunity. We do not lose our salvation, but surely we have to make things right and have our life in Christ reflected in reality and true righteousness. I know King David was not 'born again', but he was a believer who sinned. Ps. 32; 51 are still models for OT and NT believers of a broken heart, confession, repentance, and restoration. Against you O God we have sinned. There was sin in the Corinthian church among believers. Paul did not sweep it under the carpet, but called them to godly repentance and righteous living (cf. Eph.).
I trust my reflections on the holiness of God, sinfulness of man, and reality of the Christian life do not automatically make me a reprobate unbeliever. Even if my theology is a work in progress, this does not mean I cannot love God with my whole heart, mind, and soul. I have always had high academic marks. This does not count for much, but if I cannot fully articulate your view, what do we do with the unsophisticated, uneducated new believer? Have they no hope of salvation because they cannot understand how to live a godly life when they struggle with the flesh and temptation? Is trying to understand the work of Christ and victorious Christian living damnable? Is one better not to wrestle with these issues and simply trust Christ and hope for the best? Does more reflection risk the validity of one's initial simple faith?:help: