Paul was showing the gentiles in the church.... who had been considered second class people by the Jews. That they can now come up to the same high standards of Jews who had benefitted in the past by living in God's system for their lives.
For the Jews when living under the Law? They lived superior lives when compared to the pagan gentiles.
What Paul was showing the Gentiles when citing Rom 1:17.. was how at one point in the past, that the Jews had fallen into a degenerated way no better than what the typical Gentile saw in his culture ..
The Jews had fallen into a way of life that resembled just as badly how many gentiles lived before salvation. That Jews in their raw state, were no better than the typical gentile by nature. For, as Paul cited about the Jews having at one point their history, than not even one righteous one was to be found! Showing gentiles who could suffer from an inferiority complex in the presence of Jews, even Jews in the past had sunk to the same level as they, even just as bad as what the typical Gentile had been living in before saved!
Paul was saying:
"There are none righteous, none that do good?" See that, gentiles?
The Scriptures itself says that Jews, when left to their own emotions and thinking outside of the Law? That they can be just as low as the Gentiles who had been (up to then) living without God!
Paul was laying a level playing field to keep Jews from taking a superiority attitude to the "goyim" who were now accepted by God into the Kingdom.
Gentiles who were living without God's Law were in many ways gross and degenerate in their practices that even justified the Jews
in their looking down their noses at them.
But, Jews with Christ in their hearts were no longer to be that way towards the gentile! Look you Jews!
You too were at one time just as bad!
Here you are Jews! Stop thinking you are inherently superior!
For with God in your life you are equally bad!
To the choirmaster. Of David.
The fool says in his heart, “There is no God.”
They are corrupt, they do abominable deeds;
there is none who does good.
The Lord looks down from heaven on the children of man,
to see if there are any who understand,
who seek after God.
They have all turned aside; together they have become corrupt;
there is none who does good,
not even one. Psalm 14:1-3
Paul was telling the Gentiles by quoting from the torah...
See how low a Jew could sink? He is not intrinsically better than a typical gentile.
Its God's influence that made them superior!
That made gentiles realize that it was God's influence that made the Jews superior,
and not simply because they were naturally superior.
That if Gentiles submit to God's Word for their lives, they too could achieve a superior way of life for themselves!