You forgot to finish .. to bring it into context...
30 since there is only one God, who will justify the circumcised by faith and the uncircumcised through that same faith. 31 Do we, then, nullify the law by this faith? Not at all! Rather, we uphold the law.
You either have to
condmen by law, as the context dictates for 3 chapters to uphold it, or you must realize law in 31, meant the OT.
How does the faith,
in the question of 31, jew and gentile, uphold Mosaic law?
It does not, but if you realize law in 3:31, meant the OT, where Paul went into the OT, what does the OT say,
4:3, to prove justifiying faith, of Jewish David, and gentile abe, was all along, whereby upholding what was in the
OT. Law can mean OT, also, as per John 10 and 15, and eslewhere.
Unless you can show, how Rom 4, where he answered the question of 3:31, upholding the OT, proves mosaic law, justifying jew and gentile.
Look at the question.

Then look where Paul goes to prove the point,
in the OT , to show it, he did not go into Mosaic law to uphold anything, did he? No, he went into the OT, OT means law also, it was not Moses.
31
Do we then overthrow the law by this faith? By no means! On the contrary, we uphold the law.