When Claudius Caesar evicted the Jews from Rome, the Apostle Paul hatched a plan to start a Church in Rome. He went throughout the church, recruiting people to be trained in Ephesus, to in and plant the Church. Paul did not found the Church, rather he had a group called “The Seven” (Acts 21:8, 1 Timothy 1:3, 2 Timothy 4:12) that went and started the ministry. That was when Paul penned the Epistle to the Romans.
The Epistle the Romans is Paul filling in the gaps the Human history for both Jews and gentile believers who were not familiar with the state of humanity. Chapter 1, 2 and half way through chapter 3, is the state of humanity before the Cross. Paul Concludes that the outright heathen (Romans 1:19-32), the religious hypocrite (Romans 2:1-15) and the Jews (Romans 2:16-29) were all guilty and under sin.
In Chapter 3, after a brief mention of the benefits to the Jews after bashing them, he continues onto to form this conclusion.
Romans 3:9 What then? are we better than they? No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin;
10 As it is written, There is none righteous, no, not one:
11 There is none that understandeth, there is none that seeketh after God.
12 They are all gone out of the way, they are together become unprofitable; there is none that doeth good, no, not one.
13 Their throat is an open sepulchre; with their tongues they have used deceit; the poison of asps is under their lips:
14 Whose mouth is full of cursing and bitterness:
15 Their feet are swift to shed blood:
16 Destruction and misery are in their ways:
17 And the way of peace have they not known:
18 There is no fear of God before their eyes.
Then Paul starts to preach about what is true NOW, because of what Jesus has done.
Romans 3:21 BUT NOW the righteousness of God without the law is manifested, being witnessed by the law and the prophets;
22 Even the righteousness of God which is by faith of Jesus Christ unto all and upon all them that believe: for there is no difference:
23 For all have sinned, and come short of the glory of God;
24 Being justified freely by his grace through the redemption that is in Christ Jesus:
25 Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;
26 To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus.
He much later Paul emphasizes the point of all this, in Romans 11:32 For God hath concluded them all in unbelief, that he might have mercy upon all.
Hope that helps.