Sin and sins are not the same. Sin is what we are. Sins are what we do. This may sound harsh but stay with me for it is the fundamental principle of understanding why we need salvation.
In our present age, we have taken the sting out of our condition and smoothed over our true state of being by softening the truth about our sin nature. In the Garden of Eden, mankind disobeyed God and ate of the forbidden fruit. Sin was introduced in the very being of mankind and every human born of man and woman has inherited the sin nature. This is the root of sin. Sins are the acts perpetrated by this sin nature. In order for redemption to be truly effective, someone had to deal with the sin nature of man.
In the Old Testament, the sacrificial system covered sins and made retribution for the sin nature but it did not take it away. Each sacrifice was like a IOU which looked forward to the death of Christ to validate them. At the cross, all sins were forgiven and because Christ died not just for sins, but for Adam’s original sin, we can look backward to his death and know that our sins and our sin nature have been dealt with.
Today, we nullify the need for the cross by dismissing our true nature. Yes we were created in the image of God and yes, have his stamp upon us but to say we are just “good people” who have made a lot of mistakes is to say we don’t need to be redeemed from the source of our sins, because we are basically good. The root of sin had to be eradicated. Jesus died for Adam’s sin and was raised for Adam’s justification. And because we all came from Adam, we inherited his sin and but also his redemption.
Paul explains it :
So then as through one transgression there resulted condemnation to all men, even so through one act of righteousness here resulted justification of life to all men. For as through the one man’s disobedience the many were made sinners, even so through the obedience of the One the many will be made righteous. Romans 5: 18-19