People sin.
Before you believed in Jesus, what sins did GOD know you would commit in life? Now parallel the looking upon the brass snake with what Jesus did on the Cross ... when you believe He is your Savior (when you trust His Promise to be your Savior) what sins does GOD not know you will commit in the rest of your life? ... Yes, when He cleanses you with HIS blood, He cleans away ALL your sin nature ... as JESUS said, no one can snatch you from His two hands, so there is no sin you can commit that will ‘un save’ you once HE cleanses your sin nature before GOD ... God does not see your sin because He has covered them with HIS Perfect Holy Blood, All of them.