The core of Christianity is "All have sinned and come short of the glory of God"--therefore, because you are a sinner, you have failed God, therefore you need redemption in order to be accepted back into God's grace.
But I don't truly believe I am a sinner. I acknowledge that I screw up, that I make mistakes, that I fail in my goals, but I'm not a liar, I don't cheat, I don't kill people, I don't steal, I don't live a bad life.
Yes, I lose my temper once in awhile, I don't want to share my time with my children on occasion, I get tired and cranky, I spend a bit too much money on myself, etc. But are these bad, wicked, evil sins? I don't think so.
So, what or who defines sin?
When I lived in the group, it was not a sin for me to have sex with a married man, or outside of my own marriage. But if I were to do that now, it would be a sin to me because it goes against my moral beliefs. But in the group it would have been a sin to not tithe, whereas now, I don't consider it a sin and I never do it.
If I were a man growing up in a Muslim society, or Hindu, it would be perfectly fine to have more than one wife, but not according to my beliefs now. If I were part of a tribe in Africa perhaps cannibalism would be accepted in my belief system, but if I were to eat some enemy now then it would definitely be a sin.
If I were a Jew it would be a sin to eat any of the foods on their long list of non-kosher items, but because I'm not, it is fine to eat what I like. And etc..
So, who or what defines 'sin', and where does a mistake or error in judgement become a sin?
Which is why I cannot truly say in all faith "I am a sinner" because I believe that I live according to my personal morals and abide by the rules of the society I am in.