Paul's argument from Romans 1:13 through to 8:39 is that 'Jews and Gentiles alike are all under sin' (Rom. 3:9).
Yes. Although you are mixing up arguments (Paul's argument in Galatians with hos argument in Romans, which contributes to your mistakes) you are absolutely correct that Jews and Gentiles alike are all under sin.
What you fail to grasp is that Jews and Gentiles were not both transgressors of the Law.
Paul explained this very clearly, which is why it is important to read epistles rather than extract verses from them. You miss the context.
Consider Romans.
There will be tribulation and distress for every soul of man who does evil,
of the Jew first and also of the Greek, but glory and honor and peace to everyone who does good,
to the Jew first and also to the Greek. For there is no partiality with God. For
all who have sinned without the Law will also perish without the Law, and
all who have sinned under the Law will be judged by the Law; for it is not the hearers of the Law who are just before God, but the doers of the Law will be justified...
Now we know that whatever the Law says, it speaks to those who are under the Law, so that every mouth may be closed and all the world may become accountable to God; because
by the works of the Law no flesh will be justified in His sight; for through the Law comes the knowledge of sin....
For the promise to Abraham or to his descendants that he would be heir of the world was not through the Law, but through the righteousness of faith. For if those who are of the Law are heirs, faith is made void and the promise is nullified; for the Law brings about wrath, but where there is no law, there also is no violation.
For this reason it is by faith, in order that it may be in accordance with grace, so that the promise will be guaranteed to all the descendants, not only to those who are of the Law, but also to those who are of the faith of Abraham, who is the father of us all, (as it is written, “A father of many nations have I made you”) in the presence of Him whom he believed, even God, who gives life to the dead and calls into being that which does not exist....Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men, because all sinned—
for until the Law sin was in the world, but sin is not imputed when there is no law. Nevertheless death reigned from Adam until Moses, even over those who had not sinned in the likeness of the offense of Adam, who is a type of Him who was to come.
We are Jews by nature and not sinners from among the Gentiles; nevertheless
knowing that a man is not justified by the works of the Law but through faith in Christ Jesus, even we have believed in Christ Jesus, so that we may be justified by faith in Christ and not by the works of the Law; since by the works of the Law no flesh will be justified....For as many as are of the works of the Law are under a curse; for it is written, “Cursed is everyone who does not abide by all things written in the book of the law, to perform them.” Now that no one is justified by the Law before God is evident; for, “The righteous man shall live by faith.”
However, the Law is not of faith; on the contrary, “He who practices them shall live by them.”...What I am saying is this: the Law, which came four hundred and thirty years later, does not invalidate a covenant previously ratified by God, so as to nullify the promise. For if the inheritance is based on law, it is no longer based on a promise; but God has granted it to Abraham by means of a promise. Why the Law then? It was added because of transgressions, having been ordained through angels by the agency of a mediator, until the seed would come to whom the promise had been made.
Now a mediator is not for one party only; whereas God is only one. Is the Law then contrary to the promises of God? May it never be! For if a law had been given which was able to impart life, then righteousness would indeed have been based on law. But the Scripture has shut up everyone under sin, so that the promise by faith in Jesus Christ might be given to those who believe.
But before faith came, we were kept in custody under the law, being shut up to the faith which was later to be revealed. Therefore the Law has become our tutor to lead us to Christ, so that we may be justified by faith. But now that faith has come, we are no longer under a tutor. For you are all sons of God through faith in Christ Jesus. For all of you who were baptized into Christ have clothed yourselves with Christ. There is neither Jew nor Greek, there is neither slave nor free man, there is neither male nor female; for you are all one in Christ Jesus.And if you belong to Christ, then you are Abraham’s descendants, heirs according to promise.
By starting with a theory and then going to God's Word to extract verses that you can use to support that theory you create an entirely different gospel - one that reduces God's righteousness to simple jurisprudence. You miss the gospel of Jesus Christ entirely.
But if you leave Scripture intact, keep Paul's argument together rather than viewing the biblical text as reference material, you quickly realize that sin is more pervasive and perverted than moral infractions. The Law demonstrated man's unrighteousness. No flesh is justified under the Law. All men under the Law become a curse (even Jesus, as He was hung on a tree).
Try taking Paul's words that sin reigned even before anybody was under the Law. This will help you as you go through these passages.
