In verse 2 Paul continues his introduction and declares that he was not about to add anything new. His doctrines were in accordance with the acknowledged oracles of God. Though they might appear to be new, he regarded the gospel as entirely consistent with all that had been declared in the Old Testament and not only consistent, but actually promised there.
The reason why Paul was so anxious to show that his doctrine coincided with the Old Testament was because the church at Rome was made up in part of Jews. He wished to show them and the remainder of his countrymen that Christianity was built on the foundation of their prophets, and their acknowledged writings. So in doing this he would disarm their prejudice, and furnish a proof of the truth. It was a constant position with Paul that he advanced nothing but what was maintained by the best and holiest men of the nation.
Paul had never been to Rome. He was personally a stranger and it was proper for him to show his regard for the doctrines of the prophets. So, he appeals often to the Old Testament and defends every point by the authority of the Bible.
The reverence which Paul showed for the Old Testament, he never undervalued it, he never regarded it as obsolete or useless, he manifestly studied it. Paul believed that since these things were promised and predicted in the Old Testament, then Christianity is true.