Paul reminds his readers that his message could be found in the Old Testament. It is an interesting study to consider the various passages from the Old Testament that Paul refers to in his various letters and he will mention some later in the Book of Romans. If you were to be given a piece of paper, could you write down some Old Testament passages that he cites throughout his letters?
Where in the Old Testament is the gospel mentioned? The first is in Genesis 3, with the promise of a Champion who would destroy the work of the serpent (the devil). After that reference, there are countless promises and prophecies about the coming Rescuer. No doubt passages such as Isaiah 53 will come to mind. Jesus himself explained to the two despondent disciples on the way to Emmaus how the Old Testament spoke about him, and they described the effect as causing their hearts to burn with love (Luke 24).
What is the point of mentioning those promises? The answer is that they reveal that God knew the future, that he had planned a supernatural way of salvation for sinners, and that people who read or heard those predictions should have depended upon the Saviour they described. When did God’s gospel begin to be preached? Right at the beginning of human history when he announced in Eden after the Fall that a Deliverer would come. Therefore, there should have been converts from the beginning because God had told enough about the coming Saviour for them to look ahead and trust in him when he came.
Christians should learn to value the Old Testament because in it they will find Jesus. He is to be seen in its prophecies and promises as well as in the ways that the Levitical sacrifices pointed to his atoning death. So we should ask God to reveal Jesus to us in those books and perhaps read Isaiah 53 just now.