In 3:9-20 Paul brings this large section (1:18-3:20) to a close. His purpose in this section is to make it unquestionably clear why the Gospel is as he described it in 1:16,17. Remember how he described it:
This concept of the Gospel rules out common human ideas:
In 1:18-32 Paul taught that the wrath of God is being poured out on everyone because no one has lived up to the knowledge of God revealed in the created world, but rather, mankind has rejected, corrupted and distorted that knowledge with the result that there is gross ignorance of God with the accompanying degradation of the human that goes hand in hand with the rejection of God.
In 2:1-3:8 Paul has taught that the Jews' presumption that they are a privileged people and therefore not liable to condemnation is false. Their possession of the law, of circumcision and of the written revelation of God puts them in a position of greater responsibility and culpability. Their election by God was for a task, not for privileged immunity.
Paul concludes:
That, says Paul, is what God thinks of us, whether we are Jew or Gentile. It is true that if there happened to be a person who obeyed the law fully (whether a Jew by knowledge of the Law, or a Gentile without knowing it), that person would be acquitted by God. For such a thing to actually occur, however, is an impossibility, because in God's sight all are guilty, as expressed in the verses above.
Paul goes on to say that every thing the law says, it says to those 'who are under the law', that is, to the Jews, so that they too, along with the Gentiles, will be aware of their accountability and lack of legal rightness in the presence of God:
When Paul says 'every mouth' and 'the whole world' he does so to stress the universality of sin and judgment. No one race is excluded. Both Jews and Gentiles are under condemnation. Whereas the Jewish Christians supposed that their possession of the law, of circumcision and of election distinctively set them apart from the obviously lawless and pagan Gentiles, even from the Gentile believers, Paul in these words comes to the climatic conclusion to which he has been building from 1:18: that the Law, rather than identify the Jews as more righteous than the Gentiles, is actually the very thing that identifies their condemnation along with the Gentiles. Every mouth, including the Jews', is silenced in the presence of God by the Law of God. The whole world, including the Jews, is accountable to God. No one, not even the Jews, will be declared righteous in God's sight by observing the law. Rather, through the law, we, including us Jews, says Paul, become conscious of sin.
Here in these verses, 3:9-20, Paul clearly outlaws any misinterpretations we may have been tempted to make as we struggled through his arguments in chapter two. Here the possibility of anyone keeping the law is negated. In theory, and indeed in truth, if a person were to keep 100% of the law 100% of the time, that person would gain access to God's presence on the basis of that performance. In practice, Paul says, no one does it, and therefore we are all silenced by the law. The law strikes us dumb with its revelation of our lack of godliness, with its identification of our depravity and rebellion, with its statement of condemnation from which not one of us can escape. We know, and the law makes sure that we know, that we have forfeited our legal acquittal. We are all declared guilty.
Note well: the law is not a checklist against which we tick off our achievements and our performance, and tell ourselves and God how good we are. Rather the law accuses us; it tells us that Paul is right in his description of every human being in Romans 3:10-18, and shows us that we all stand condemned as sinners in the presence of God.
Thus Paul has proved how necessary the Gospel, and how necessary it also is that we have the right understanding about the impossibility of our performance ever meriting our acceptance with God.