God has given you a CONSCIENCE.
"Con" means with and "Science" means knowledge.
So every time you sin, you sin with knowledge. You know its wrong!
You don't just do wrong things, you do them willfully. And your conscience condemns you. Your conscience is like a judge that sits in the courtroom of your mind to condemn or justify you.
The Bible says that God's law is written on our hearts because sometimes our conscience justifies us and at other times it accuses us (Romans 2:15).
Then there is all the sin that you do that you don't know about because you do it in ignorance or with a seared (hardened) conscience.