The problem is that he really didn't have a biblical perspective of sin. Erickson lists 5 perspectives on sin that he sees in the Bible. Sin is an inward inclination. Sin is not merely wrong acts, but sinfulness as well. It is an inherent inner disposition inclining us to wrong acts.
Sin is rebelliousness and disobedience.
Sin entails spiritual disability. It alters our inner condition, our character. In sinning we become twisted or distorted as it were. The image of God in which we were created is disturbed. Sin is incomplete fulfilment of God's standards.