Romans 1:2

Paul next began to exalt the gospel that God had called him to proclaim. It was a message that God had promised, not just prophesied, in the Old Testament Scriptures. The words "his" and "holy" stress the unique origin of the gospel. God had inspired the Old Testament by speaking through men as He gave His revelation. Paul did not preach an unanticipated gospel but one that God had promised through His prophets (cf. Romans 4:13-25; Romans 9:4; Romans 15:8). This is the reason Paul appealed to the Old Testament so fully in this and other of his epistles. Specifically, Paul’s gospel was not a human invention that tried to make the best of Israel’s rejection of Jesus Christ.