Paul alludes to Old Testament precedents and now he must flesh this out. He does with Abraham (Genesis 15:6) and David (Psalm 32:1-2) citing that their justification had nothing to do with what they had done, but rather because of their belief.

Note that both of which are passages connected by the word "reckon," and thus used together in an example of the Jewish practice of gezerah shavah, "equal category."

Also note the word "reckon" was an ancient bookkeeping used in business documents that meant to "credit to one's account." This explains the use of Paul's example of wages and work in 4:4-5. The Jews would have seen this as more than legal or business transaction though. When God speaks, He creates a "new reality." The old is gone and the new has come.

And finally note that Paul has now adroitly shown that justification comes through faith using all three sections of Tanach (Abraham: the Torah; David: the K'tuvim; and Habakkuk: the Nevi'im (see Romans 1:17).

Though Paul refers to Abraham as "our forefather according to the flesh," thus seeming that the "our" refers to the Jews, Paul later changes the readers perspective on this (cf. 4:11-12).

Verse 5 gives us a great name for God: "Him who justifies the ungodly." At face value, this seems to be a shocking contradiction of terms, especially considering Romans 2:5, a view of the righteous judge "who will render to each person according to his deeds." The Old Testament paints a very dark, ugly picture of unjust judges who declare the unrighteous as righteous. In fact God Himself condemns unjust judges in Exodus and of Himself declares in Exodus 23:7, "I will not acquit the wicked," which might as well be translated, "I will not justify the ungodly." In fact the LXX of Exodus uses the same Greek terms of what God will not do as Paul uses to say what God will do! Paul however does not yet fully answer this scandalous truth can be so. He hinted at in in 3:21-22, but that was not even close to a full explanation. All Paul has done thus far is told us in 3:25-26 who this does not actually affect God's own character. And Paul is also implying (and will spell out later) that the ungodly should stop being ungodly; however, their justification is not based on any foreseen changing of their ways.

Romans 4:1-8