First, because the ‘works’ to which the passage clearly refers are the works required for covenantal acceptance, i.e. circumcision. Once the issue being discussed, i.e. of the status of the Gentiles corporately, has been identified, there is no suggestion in this text that Paul is talking about someone trying to gain acceptance through moral achievement. Second, the accounting meaning has violated the covenantal context of Genesis 15, Romans 3–4 and Galatians 3. I would argue that covenant is the
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