Final answer:
Paul shows that 'justification by grace' is not a new idea, but an OT concept by referencing Genesis and Abraham's faith.
Step-by-step explanation:
Paul shows that 'justification by grace' is not a new idea, but an OT concept by referencing Genesis and Abraham's faith. In Genesis, Abraham believed in God's promise of making him the father of many nations, and his faith was counted as righteousness. Paul refers to this in Romans 4 to demonstrate that justification by faith existed in the Old Testament. This shows that the idea of 'justification by grace' predates the New Testament.