"It was confidence in God, a positive acknowledgment of God’s power as creator, a calm certainty that God had made known to Abraham his purpose and could be relied on to perform it without further question or condition. Here from another aspect is the same reason why Abraham’s faith should not be thought of in terms of covenant loyalty or as incomplete apart from works, for faith is confidence in God’s loyalty as alone necessary, as alone able, as alone sufficient to bring God’s promise to full effect."
James D.G. Dunn, Romans, Word Commentary, (1990) 1:239 [italics mine].
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Dunn is a leading British New Testament scholar whose work has contributed to the so-called 'New Perspective on Paul' (NPP) that originally derives from E.P.Sanders's claim that the Reformation tradition, from Luther onwards, had mis-read Paul on Judaism and the relation between faith and works. The NPP has been cited by some former Lutherans and Reformed as influential in their decisions to convert to Roman Catholicsm, on the grounds that the NPP's reading of Paul shows that the Roman Catholic view of salvation as a process that includes growth in justification/sanctification through good works correctly interprets Paul's thinking. Yet here, in his commentary on Romans 4, Dunn is saying the opposite - Abraham's faith - cited, of course, by Paul, as paradigmatic for the Christian's justification by faith alone - was "alone sufficient to bring God's promise to full effect".
We need only add the caveat that faith is instrumental, not causal when it comes to the justification of the sinner; i.e. faith is the open hand that receives God's grace in and through Christ, it is not meritorious in and of itself (lest we have something in which to boast!).
Soli Deo Gloria!