Paul's microcosm of the Gospel is found in Romans 1:17. It takes six words in the English language (though he only needed five in Greek): "The righteousness of God is revealed."
In the previous verse, he had just said that the Gospel is the power of God for salvation. Then in these few words he sums up how that can be. He starts v.17 with the word "For..." to alert us that he is about to tell us is the secret that has been revealed that is the basis for his confident claim. The story of the success of the gospel is summarized as "The righteousness of God is revealed."
As the letter proceeds, he will point out how desperately we need the righteousness of God. Without it, we have no hope. However, we need to understand what the righteousness of God is in contrast to two quite distinct aspects of our hopelessness.
Firstly, God's righteousness is the antidote to human unrighteousness. Paul will be clear - the problem with the world is human sin. So God's solution is in the provision of his righteousness as a free gift of grace in his Son Jesus. He gives us the righteousness that we don't have. So much we mostly understand; but Paul is far more radical than that.
Secondly, God's righteousness is the antidote to human righteousness. Paul has two targets in mind. The Gentiles are sinners, and their catalog of sinfulness is laid out in Romans 1 in all its ugly expressions. The religious people are saying loud "Amen's" throughout: unrighteous people like that need God's righteousness. But then Paul turns his guns on the religious people: the righteous law-abiding ones. They too need God's righteousness - not just to replace their human unrighteousness, but also to replace their human righteousness.
As Isaiah had pointed out, all our righteousness is as "filthy rags" (Is 64:6). The term refers to soiled menstrual cloths, but we could perhaps change the metaphor to the oily, greased-stained rags lying around the floor of a car mechanic's workshop. Out righteousness is the equivalent of a mechanic trying to clean himself up at the end of the day by wiping the dirt off with one of those rags. All he is doing is smearing the grime around.
God has an issue with us: not just with our unrighteousness, but with our righteousness too! Remember Isaiah. He knew he had unclean lips. But the context was not him saying some things he should not have said when he banged his finger with a hammer in the workshop. That would have been the unclean lips of his unrighteousness. However, he was, in his vision, worshiping the Lord as a priest in the temple, not wielding a hammer as a mechanic in a workshop. If he had said anything it would have been an "Amen" to the "Holy holy holy" song of the angels. It was his righteousness, his religious attempts to find access into the presence of a holy God, that he saw was soiled and filthy.Our praise songs and our prayers are as unclean as the profanities of the clumsy pagan hammer wielder.
Isaiah became a prototype of good, upright, moral religious people who need the gospel every bit as much as the profane pagans. If anything, such people might be worse off - because they do not see that they have a problem. They need God's righteousness to cover their human righteousness at least as much as others need God's righteousness to cover their human unrighteousness. And that's Paul's summary of the gospel. A story in six words: "The righteousness of God is revealed."