The Promise of the Gospel—Romans 1:2
"The gospel he promised before hand through his prophets in the Holy Scriptures..." (Romans 1:2)
If you look at the account of Paul's life in the Book of Acts, you'll see he was constantly defending the gospel before masses of people, Jewish leaders, the Roman government, and even before the original Apostles who learned directly from Jesus. I think this was because the gospel was so radically different than anything that had previously been taught and so the natural tendency was to question it.
Paul's eyes were opened, and he saw everything the law and the prophets promised—they promised Jesus!
Here's a beautiful truth: the very same gospel that Paul preached was promised throughout the entirety of the scriptures. Even today, we have a hard time understanding this, as I imagine the Apostle Paul did at first. In his own words, he admits to being faultless concerning legalistic righteousness (Phil. 3:6)—meaning he followed the letter of the law, which he knew inside and out and lived every moment making sure he was doing all that the law required. Some things don't change, for example, how many of us are like this today?
Imagine how mind-blowing it must have been to encounter Jesus and then to start piecing together everything the law and prophets talked about, which included everything Paul previously misunderstood while trying to do all that the law required. Imaging waking up to the fullness of what those things actually revealed—Jesus! This incredible revelation was so profound that Paul immediately set off for the desert of Arabia and spent three years sorting it out (Gal. 1:11-12, 17-18).