Sin is so often spoken of in general, vague terms and we've been preached "tolerance" to the degree that almost anything is acceptable, We have also been taught to rationalize sin away: "I didn't know any better," "I'm a victim of a poor upbringing," "I'm really a good person inside - it was just a moment of weakness."  Let us not be like the Pharisee in Lk.18, who compares himself with others as his standard, but let us be like the publican who smote his breast and wouldn't even lift his eyes to heaven, so ashamed he was by his sin.  "A sin consists in doing, saying, thinking, or imagining, anything that is not in perfect conformity with the mind and law of God. Sin is 'transgression of the law.' The slightest, outward or inward departure from the absolute mathematical parallelism with God's revealed will and character constitute a sin, and at once makes us guilty in God's eyes." J.C.Ryle
     Does this sound like an impossible standard? It is for fallen man. But the Lord Jesus makes it possible for us. Christ, through the Holy Spirit, circumcises our heart. This is the "putting off of the body of the sins of the flesh," it is "circumcision of the heart, in the spirit," it is "circumcising the foreskin of the heart(opening up the heart)." Col. 2:11, Rom. 2:29, Duet. 10:16  This act of Christ in the heart prepares and enables us, and works to purge away a multitude of sins.
     There are 3 kinds of sin: sins of commission, sins of omission, and sins of ignorance. Sins of commission are doing those things that we should not. God's law  defines the things that we should not do. Like, lying, coveting, or adultery. But obeying such commands outwardly is not enough. Matt.5:22,28 makes it quite clear that our inner thoughts are sinful as well and must also be conformed to the likeness of Christ. We must "bring into captivity every thought to the obedience of Christ."  Sins of commission may also be called presumptuous sins - sins we commit willfully. Willful sin, as a pattern, is dealt with very harshly in Scripture, because the person knows better. "For if we sin willfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins." Heb.10:26. See Prov.13:13, Ezk.18:24.
     Sins of omission are leaving undone the things that we ought to do. These sins are the "other side" of the sins of commission. It's easier not to put forth the extra effort in speaking out when we should, or teaching our child the right thing instead of nothing at all. But God's word tells us to strive, abide, seek, ask; all actions. In Matt.25:41-46 Jesus severely condemns those who passed up opportunities to serve Him when they had the chance. It is a solemn warning. "To him that knoweth to do good, and doeth it not, to him it is sin."
     Then there are the sins of ignorance. If you are like me, you may have thought that God would only hold us accountable for what we know. But that is not what Scripture teaches. Luke 12:48 says: "But he that knew not, and did commit things worthy of stripes, shall be beaten with few stripes." The servant was punished for things "he knew not."  He wasn't punished as harshly as the servant that sinned willfully and in full knowledge, but he was punished none-the-less. Study Leviticus 4 & 5 and Num.15. You will see that when the people sinned in ignorance and later found out, they were to make a sin offering unto the Lord.  This may not seem fair to us, but it shows how very serious sin is to our most Holy God. We are to "keep our heart with all diligence," and "search the scriptures daily."  Ignorance does not excuse our sin.  Jesus, Himself said, "If I had not come and spoken unto them, they had not had sin: but now they have no excuse for their sin." Jn. 15:22  "When we make the measure of our sinfulness to be our own miserably imperfect knowledge and consciousness, we are on very dangerous ground." J.C.Ryle  The psalmist recognized the deception and fallibility of his own nature. We would do well to pray this verse: "Who can understand his errors? Cleanse Thou me from secret faults." Ps.19:12
     Remember, the believer is accountable to confess each sin as he knows about them. Our past sins are forgiven at salvation (Rom.3:25, Heb.8:12, I Tim1:13), but Scripture teaches that we are to continually repent as needed. See Ps.32:5, I Jn.1:19, Rev.3:19. We must not assume that our sin is automatically covered by the blood of Jesus apart from our heart-felt repentance.
     Finally, there is a needful principle to learn. Read what Ezk.36:31 says: "Then shall ye remember your own evil ways, and your doings that were not good, and shall loathe yourselves in your own sight for your iniquities and for your abominations."  At the time we confess and repent of a sin, we obtain God's forgiveness. But later, sometimes years later and sometimes often, we remember the sin again with great pain and sorrow. We realize anew, "against Thee, Thee only, have I sinned, and done this evil in Thy sight."  Why do we feel so terrible again? Have we not already obtained forgiveness? Yes, but the difference is you now know God and His ways much better, you are more mature in your faith. You see more clearly the terrible sinfulness of your sin in the sight of God. I consider these remembrances a gift. When we experience this we do feel a loathing for ourself and for the sin we committed against God. This creates a deeper reverence for the Father and a deeper hatred for sin. It encourages us to obey His commands and to sit humbly and learn at His feet.