"...the gospel he promised before hand through his prophets in the Holy Scriptures..." (Romans 1:2)
All the writings of the Old Testament, all the law and all the prophets foretell of the gospel of God or good news of Jesus Christ. The gospel is the epicenter of history past and history future. It is not by accident that Paul starts off this brilliant expose to the Romans with the claim that this good news is one and the same of which the prophets had been foretelling for centuries.
Many marvel and are caught off guard concerning this - many believe that the new covenant or good news of Jesus came about because God just got feed up with the first system of laws and sacrifices and decided to do something entirely new and different. But the truth of the matter is that this was always the plan, the plan before creation, the plan before man and the plan before fall. In fact it is Peter who tells us in I Peter chapter 1, that it is "not with perishable things such as silver or god that you were redeemed from an empty way of life handed down to you from your forefathers, but with the precious blood of Christ, a lamb without blemish or defect. He was chosen before the creation of the World, but was revealed in the last times for your sake" ( I Peter 1:18-20 ). Peter clearly teaches us that Jesus was chosen to bear the penalty of our sin before that sin existed. That is why God told the serpent in the garden, "he will crush (or strike) your head, and you will strike his heel" ( Genesis 3:15 ). Right after sin entered into the world, God told Satan that there was one coming or there was one already on the way to undo what he did, and that was Jesus!
Mankind's sin in no means caught God off guard. This didn't send the Lord back to the drawing board trying to figure out another plan after the first one had been foiled. No, Peter says that Jesus was the Lamb chosen before the foundation of the world! And Paul is telling us in the beginning of Romans that this gospel is what was promised before hand through the entire Holy Scriptures. This is the good news of the grace of God to which he was called to be an apostle.
God wasn't doing something new because he got feed up with the first system. Hebrews says that the law was types and shadows of what was to come ( Hebrews 10:1 ). Meaning the law always pointed towards Jesus, the law brought about the conscience of sin indicating the need for a Savior. This Savior is in the person of Jesus Christ and through faith in His blood which he offered for the sins of the entire world ( Hebrews 9:28, I John 2:2 ). For those who believe, this grace is a sweet and precious gift. Sins are completely covered and completely paid for through the redemption that came through Christ's atoning blood.
This was always the plan, this was always the way and this good news is what the prophets foretold and promised through the Holy Scriptures. And this good news is what Paul was set apart for. This good news is what will save us and give us abundant life if we believe those prophets and Paul and believed in Jesus as our Savior, to the Glory of God.