Imagine you're trying to convince someone their deeply held belief is wrong. Paul does this masterfully here, starting with a concession that sets up a powerful 'but.'
Paul anticipates the argument the Jewish readers might make: 'Abraham, our great ancestor, was righteous. Surely he was justified by his actions!' If this were true, Paul concedes, Abraham would have a legitimate reason to boast about his achievements. However, Paul immediately pivots. The crucial question is not whether Abraham could boast before people, but whether he had grounds to boast before God. The text implies that whatever pride might exist among men, it evaporates in the holy presence of God.
This is Paul's strategy: He doesn't directly deny Abraham's greatness but redirects the focus to the divine perspective. The 'if' in the verse is a hypothetical, a concession for the sake of argument, not an agreement that Abraham was justified by works.