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.
Some may wonder why I have chosen to address this topic prior to engaging with other topics (such as the meaning of the ‘days’ in Genesis 1). I have done so because many young earth creationists consider this to be the more pressing concern, and perhaps even the strongest argument for their position from Scripture.