1 Peter 1:16 for it is written: "Be holy, because I am holy." (NIV)
Sin in New Testament Greek is hamartia, amartia. It means to miss the mark, to err, or be mistaken.

Levels of sin
So we will not be "hardened by the deceitfulness of sin" Hebrews 3:13, we need to understand the levels of sin and how we fall from one level to the other.

1. Unintentional sin
Unintentional sin is when you didn’t realize what you have been doing is sin. This will be repented of as soon as the Christian learns of it.
There are three types of unintentional sin:
Falling short or making mistakes is inevitable for people in this life. 1 John 1:7-8
Presumptuous sins are when Satan fools us into thinking we are doing what is right, when we are actually sinning. Psalms 19:13
Unknown sin Leviticus 4:2

2. Reoccurring sin Romans 3:9; 4:8; 6:14; 7:8-25; 8:2,10 1 John 1:7,8

Reoccurring sin is where you realize it is sin, and you don’t want to do it, but in times of emotional stress you can’t help yourself. Example pride, gossip, cursing, back biting (taking out your anger on your spouse or kids.)...
Romans 7:18-20 says: For I know that nothing good dwells in me, that is, in my flesh. I can will what is right, but I cannot do it. 19 For I do not do the good I want, but the evil I do not want is what I do. 20 Now if I do what I do not want, it is no longer I that do it, but the sin that dwells within me. (NRSV)
The Holy Spirit will conform us to the image of Christ as we go through life, but it takes time.
Reoccurring sin is not considered willful because you can’t willfully sin when your trying not to. This is called "Walking in the Spirit."

3. Willful sin "carnal Christian" 1 Corinthians 3
Willfully committing sin is when you know you shouldn’t do something but do it anyway. It is God given desires taken out of bounds. It is not fighting against God but simply satisfying you desires in the wrong way.

1 John 1:9 If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness. (NIV)
1 John 3:4 Everyone who sins breaks the law; in fact, sin is lawlessness. (NIV)
Willfully omitting something you know you should do is also sin.
James 4:17 says: Anyone, then, who knows the right thing to do and fails to do it, commits sin. (NRSV)
See also Romans 14:23 and Ephesians 4:26
1 John 1:9 says: If we confess our sins he who is faithful and just will forgive our sins and cleanse us from all unrighteousness. NRSV

4. Unbelief Hebrews 3&4; 10:26 and Rebellion 1 John 3:9
A person at this stage either falls into unbelief, doesn’t care any more, or Rebellion, fighting against God’s kingdom.

5. Apostasy Matthew 12:31-32
The Unpardonable Sin - those who said "no man can do what Jesus did except God be with him.", are the very same who attributed Jesus miracles to Beelzebub. Jesus then said blasphemy of the Holy Spirit will never be forgiven. The unpardonable sin occurs when you receive the Gospel, and through the Holy Spirit gain the absolute knowledge that Jesus is God and savior then "you trod under foot the Son of God, and count the blood of the covenant, wherewith you were sanctified, an unholy thing, and do despite unto the Spirit of grace. See Hebrews 10:10,26-31 Fully understanding the gospel and rejecting it will never be forgiven.

John 6:44 says no man comes to Christ without God’s spirit drawing him. In Romans 1:22-32 makes it clear, When we worship the creature (self) rather than creator God begins to give us up to a reprobate mind. In Genesis 6:3 Gods spirit will not always strive with man. Ephesians 4:30 the Holy Spirit can be grieved away. In 2 Thessalonians 2:10-12 God sends a strong delusion so we believe the lie and are damned.

If you commit the Unpardonable Sin the Holy Spirit will leave you and you will no longer believe. See 1 Corinthians 15:2. IF you still believe and want to be saved, then you couldn’t have committed it!

The effect of sin
All 5 levels of sin can cause major problems in life. If we continually and willfully practice sin we will eventually destroy ourselves. We must; therefore, with the Holy Spirits help, rid ourselves of all sin.

John 9:41 Jesus said to them, "If you were blind, you would have no sin; but now you say, ‘We see.’ Therefore your sin remains. (NKJ)
If it is truly an unknown sin the blood of Jesus cleanses you from it. But God knows if you are using that as an excuse, and He will chastise you! See Hebrews 12:5-11

1 John 1:6-10 says: If we say we have fellowship with him while we are walking in darkness (willfully sinning), we lie and do not do what is true; 7 But if we walk in the light as he himself is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin. (unintentional and reoccurring) 8 If we say that we have no sin, we deceive ourselves, and the truth is not in us. 9 If we confess our sins he who is faithful and just will forgive our sins and cleanse us from all unrighteousness. (all) 10 If we say we have not sinned, we make him a liar, and his word is not in us. (NRSV)

As long as we are "Walking in the Spirit," which is trying our best and trusting in Jesus, his blood continues to cleanse us form all sin. This takes care of unintentional and reoccurring sin. No one should be scared that they may loose your salvation because you weren’t careful enough.

Willful sin destroys our relationship with God and others. If we persist in it we will eventually destroy ourselves.
1 John 5:16-18 If anyone sees his brother commit a sin that does not lead to death, he should pray and God will give him life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that he should pray about that. 17 All wrongdoing is sin, and there is sin that does not lead to death. 18 We know that anyone born of God does not continue to sin; the one who was born of God keeps him safe, and the evil one cannot harm him. (NIV)

As long as they are still alive pray that they repent and restore their relationship with the LORD.
Unbelief and rebellion, spiritually, is the dying process. You can not remain in it for very long before you spiritually die.
Once you have committed Apostasy you are doomed to hell for all eternity.

Righteousness
1 John 3:4-11 Everyone who commits sin is guilty of lawlessness; sin is lawlessness. 5 You know that he was revealed to take away sins, and in him there is no sin. 6 No one who abides in him sins; no one who sins has either seen him or known him. 7 Little children, let no one deceive you. Everyone who does what is right is righteous, just as he is righteous. 8 Everyone who commits (willfully practices) sin is a child of the devil; for the devil has been sinning from the beginning. The Son of God was revealed for this purpose, to destroy the works of the devil. 9 Those who have been born of God do not (willfully practice) sin, because God’s seed abides in them; they cannot (willfully practice) sin, because they have been born of God. 10 The children of God and the children of the devil are revealed in this way: all who do not do what is right are not from God, nor are those who do not love their brothers and sisters. 11 For this is the message you have heard from the beginning, that we should love one another. (NRSV)

Once you have become a Christian you will not practice sin because your nature is to want to do right. You will not like anything that will hurt another. However, a Christian may, in times of emotional stress fall into "reoccurring sin." But a Christian can never fight against the kingdom of God. His new nature will not permit it.

Galatians 5:24-25 And those who belong to Christ Jesus have crucified the flesh with its passions and desires. 25 If we live by the Spirit, let us also be guided by the Spirit. (NRSV)

If Christians allow themselves to be guided by the Holy Spirit and set their minds on righteous and not on sin they will be walking in the spirit.

Romans 6:19-22 I am speaking in human terms because of your natural limitations. For just as you once presented your members as slaves to impurity and to greater and greater iniquity, so now present your members as slaves to righteousness for sanctification. 20 When you were slaves of sin, you were free in regard to righteousness. 22 But now that you have been freed from sin and enslaved to God, the advantage you get is sanctification. The end is eternal life. (NRSV)

This is progressive sanctification. The more you yield to righteousness the less you will sin.

When is it sin?
An emotion or desire becomes sin only when it crosses a certain line.
1 Corinthians 10:13 No testing has overtaken you that is not common to everyone. God is faithful, and he will not let you be tested beyond your strength, but with the testing he will also provide the way out so that you may be able to endure it. (NRSV)
Notice that it is possible to be tempted and not sin. The temptation itself is not a sin.

Matthew 5:21-22 "You have heard that it was said to those of ancient times, ‘You shall not murder’; and ‘whoever murders shall be liable to judgment.’ 22 But I say to you that if you are angry with a brother or sister, {other ancient authorities add without cause} you will be liable to judgment; and if you insult a brother or sister, you will be liable to the council; and if you say, ‘You fool,’ you will be liable to the hell. (NRSV)

Matthew 5:27-32 "You have heard that it was said, ‘You shall not commit adultery.’ 28 But I say to you that everyone who looks at a woman with lust (coveting) has already committed adultery with her in his heart. 31 "It was also said, ‘Whoever divorces his wife, let him give her a certificate of divorce.’ 32 But I say to you that anyone who divorces his wife, except on the ground of unchastity, causes her to commit adultery; and whoever marries a divorced woman commits adultery. (NRSV)

James 1:15 Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death.(NIV)

1. Corinthians 10:13 says that being tempted is not a sin. Neither are feelings. We should not feel guilty of feeling things as long as they are not taken out of bounds.
2. Constantly thinking about the thing is not a sin but it puts you on dangerous ground.
3. If you think "how could I do it?" and set up a Plan, it is becoming a sin.
4. You may not be able to rob a bank but if you check to see if you could pull it off, it is SIN.
5. If you try but can’t quite do it its still SIN.
6. Actually doing something wrong is of course SIN.

Luke 6:45 The good person out of the good treasure of the heart produces good, and the evil person out of evil treasure produces evil; for it is out of the abundance of the heart that the mouth speaks. (NRSV)

Romans 14:23 But those who have doubts are condemned if they eat, because they do not act from faith; for whatever does not proceed from faith is sin. (NRSV)

The Law of Faith says even if it is OK, but you think it is wrong and do it you are committing sin. Likewise, if it is sin but you honestly think it is righteousness, because you are acting in faith, God credits you as if it were righteousness. God is pleased with you faith. You probably will; however, be hurt in this life because of your sin. If you use this as an excuse for sin God knows and will chastise you. See Hebrews 12:5-11

Biblical Sins
Romans 13:9-14 adultery, murder, stealing, coveting, orgies, drunkenness, sexual immorality, debauchery, dissension and jealousy. (NIV)
1 Corinthians 6:9-10 sexually immoral, idolaters, adulterers, male prostitutes, homosexual offenders, thieves, greedy, drunkards, slanderers, swindlers (NIV)
2 Corinthians 12:20 quarreling, jealousy, anger, selfishness, slander, gossip, conceit, and disorder. (NRSV)
Galatians 5:19-21 19 sexual immorality, impurity, debauchery, idolatry, witchcraft, hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions, envy, drunkenness, orgies, and the like. (NIV)
Ephesians 4:19 Having lost all sensitivity, they have given themselves over to sensuality so as to indulge in every kind of impurity, with a continual lust for more. (NIV)
Ephesians 4:25-32 lying, stealing, Unwholesome talk, bitterness, rage and anger, brawling and slander, along with every form of malice. (NIV)
Ephesians 5:4-5 obscenity, foolish talk or coarse joking, which are out of place, No immoral, impure or greedy person—such a man is an idolater (NIV)
Colossians 3:5,8,9 sexual immorality, impurity, lust, evil desires and greed, which is idolatry, anger, rage, malice, slander (blasphemy), and filthy language from your lips, and lying to each other. (NIV)
1 Timothy 1:10-13 adulterers, perverts (Homosexual), slave traders (kidnapers), liars, perjurers, blasphemers, persecutors, violent. (NIV)
Deuteronomy 18:10-11 child sacrifices in fire, divination, an observer of times, an enchanter, a witch, a charmer, a consulter with familiar spirits, a wizard, or a necromancer. (KJV)
Proverbs 23:23 the drunkard and the glutton (NASB)
James 2 Showing partiality
murmuring, complaining.
Philippians 4:8 Worry (NRSV)

From extra biblical documents that the early church fathers used.
4 Maccabees 1:27 indiscriminate eating, gluttony, and solitary gormandizing. (NRSV)
Barnabas 20:3-5 arrogance, hypocrisy, rape, haughtiness, deceit, malice, self-sufficiency, poisoning, magic, having no fear of God, hate truth & love falsehood, don’t take care of the widow and orphan, impatience, abortion, partiality,
didache 5:2 abortion

Obsession
Obsession is also called coveting. We need to guard against becoming obsessed with anything. The following chart shows how you can be obsessed in different ways by the same thing and not realize it.