1 Corinthians 15:22 NIV84

22 For as in Adam all die, so in Christ all will be made alive. (NIV)

This verse for me speaks clearly where our sin comes from. We inherited the sinful nature from Adam. Through Adam the human race has fallen into sin and is far from God. That is why each human being is not born without sin like a clean piece of paper. While we are still in our mothers wombs we have this stain of sin in our human nature.

This is one of the reasons why I can't be proud about my life, even though I don't commit big sins (stealing, killing etc.) I am in the same boat than a killer. If sin had a visible colour like orange, we all would have been born orange. The only solution, antedote to get rid of this orange is to be washed by the blood of Jesus.

It is amazing that God does not see me as orange any more but just like Jesus, pure without sin.
We have the spiritual birthmark (Belief in the death and resurrection of Christ) by which we are made alive with Christ. A new identity, a new life.....it is free but it will cost you your whole life.