Paul 'proves' faith and establishes confession, tradition, and morality - but not as if we knew in advance what God means by these things. You have no need that a man should teach you; the anointing teaches you everything. In 3:30 above and 4:1 below, see Hays 2005.
1: What then shall we say? Have we found Abraham our forefather according to the flesh? 2: For if Abraham was justified by works, he has something to boast about, but not before God. 3: For what does the scripture say?[9a]
"Abraham believed God, and it was reckoned to him as righteousness."
[4-12: reckon (Dunn)] 4: Now to one who works, wages are not reckoned as a gift but as due.
5: And to one who does not work but trusts him who justifies the ungodly,
that one's faith is reckoned as righteousness.
6: So also David pronounces a blessing upon the one to whom God reckons righteousness apart from works: 7: "Blessed are those whose iniquities are forgiven, and whose sins are covered; 8: blessed is the one against whom the Lord will not reckon sin."
In the next 5 chapters, Paul feels on more solid ground since he can 'explain' the thesis of faith by a number of metaphors from the tradition of Israel. But also he must defend the holiness of the law - of confession, tradition, and morals.
9: Is this blessing pronounced only upon the circumcised, or also upon the uncircumcised?
We say that faith was reckoned to Abraham as righteousness. 10:
How then was it reckoned to him?[10a]
Was it before or after he had been circumcised?[10b]
It was not after, but before he was circumcised.
11: He received circumcision as a sign or seal of the righteousness which he had by faith while he was still uncircumcised.
The purpose was to make him the father of all who believe without being circumcised and who thus have righteousness reckoned to them,
12: and likewise the father of the circumcised who are not merely circumcised but also follow the example of the faith which our father Abraham had before he was circumcised.
13: [13-21: believe (Dunn)] The promise to Abraham and his descendants,
that they should inherit the world, did not come through the law
[=Torah, =circumcision, =Jew - notice how Paul seems to have used these almost in parallel to this point]
but through the righteousness of faith.
14: If it is the adherents of the law who are to be the heirs, faith is null and the promise is void.
15: For the law brings wrath, but where there is no law there is no transgression.
16: That is why it depends on faith, in order that the promise may rest on grace and be guaranteed to all his descendants
-- not only to the adherents of the law but also to those who share the faith of Abraham,
for he is the father of us all, 17: as it is written,
"I have made you the father of many nations" --
in the presence of the God in whom he believed,
who gives life (zwopoiountos) to the dead and calls into existence the things that do not exist.
18: In hope he believed against hope, that he should become the father of many nations; as he had been told,
"So shall your descendants be."
19: He did not weaken in faith when he considered his own body,
which was as good as dead because he was about a hundred years old,
or when he considered the barrenness of Sarah's womb.
20: No distrust made him waver concerning the promise of God,
but he grew strong in his faith as he gave glory to God,
21: fully convinced that God was able to do what he had promised.
22: That is why his faith was "reckoned to him as righteousness."
23: [First person plural] But the words, "it was reckoned to him," were written not for his sake alone, 24: but for ours also. It will be reckoned to us who believe in him that raised from the dead Jesus our Lord, 25: who was put to death for our trespasses and raised for our justification.
This series on Romans begins here