Is there any aspect of Christianity more brought up - and messed up - than "sin"? (possibly "works" I guess but thats another post). As usual, the scriptures are very clear on the topic. Its men who complicate things.
The bible teaching is simple - sin separates people from God (Isaiah 59:2).
Why? Can't He just overlook it? No, He can't. Sin is "darkness" (Col 1:13) and "God is light - in Him there is no darkness at all" (1 Jn 1:5). God and sin can therefore no more co-exist than light and darkness can co-exist. The presence of one banishes the other. "What fellowship has light with darkness?" asks the apostle Paul in 2Cor6:14. "None" is the answer he is obviously looking for.
When we truly appreciate this it makes every sin we commit that much more important doesn't it? Even "little" sins are still "darkness" and will separate us from the "light". If we only committed one sin in our whole life would that be enough to separate us from God for eternity? You bet! According to the scriptures you'll be just as cut off as the person with a multitude of sins. I won't quote it all here but read again about this principle in Ezekiel 33:11-19. Notice especially the end of v12: "A righteous man will not be able to live by his righteousness on the day he commits sin". One sin - any sin - is enough to separate you from a holy God.
One of the most insidious false teachings going around these days is that while all the above applies to men generally, Christians are somehow immune to the problem because the blood of Christ "automatically cleanses" them of any sin they commit regardless of what they do about it.
Thats not what the scriptures teach. In the New Testament the conditions for forgiveness of sins for Christians is the same as for non-Christians - except for baptism of course. Nowhere do you find Christians getting baptized again for the forgiveness of their sins but you certainly do see them needing to repent of their sins and confess them.
1Jn1:9 (written to Christians remember) is very clear: "If we confess our sins He is faithful and righteous to forgive us our sins and cleanse us from all unrighteousness".
What's the most important word in this verse? "IF". The necessary conclusion is that if you DON'T confess your sin you DON'T obtain the forgiveness and cleansing.
I remember a discussion about this between two Christians. One finally said to the other in disbelief: "If all this is true then I would need to be watching what I'm doing all day long and confessing and repenting regularly". Well, yes!
Or even better, do as Paul advises the Christians in Corinth and "STOP SINNING" (!) (1Cor15:34)