Romans 1:16-17 encapsulate the thesis of Paul’s entire treatise – the gospel of Jesus Christ – which will be further expounded and elaborated upon in the chapters that follow. Paul is never put to shame by the gospel, since when it is preached, people are saved. This is why he has been so eager to preach the gospel in Rome.
Paul had, at the time of his writing, never visited the Roman church in person (Rom 1:13; 15:22-29). He thus expands his salutation so as to include a creedal summary of the gospel and his apostolic calling. Presumably Paul’s intent here was to establish credibility with the recipients of his letter, whom he had not yet met with in person.