Why does Paul say the gospel was according to the Scriptures?

According to the context, Christ had to be dead to prove beyond doubt that He was dead; this was not tomfoolery or beguiling. He was dead three days and three nights in the tomb, then arose again the third day according to the Scriptures, and the Sovereign God was in complete control. The explanation notes that the unbelieving world ridicules the Book, but history and archaeology confirm the Old Testament writers lived as real flesh and blood, with excerpts of their writings found in cuneiform on clay tablets, written 1,000, 1,500, 2,000 years before Christ came. David wrote in the Psalms a perfect picture of the crucifixion and, in 1,000 B.C., concerning His resurrection, though in veiled language that took this apostle to bring out into the open. It was all back there, and that is why he mentions it: that it was according to the Scriptures that the Gospel came about, and that He arose again the third day according to the Scriptures. That is the Gospel Paul received and took to Corinth. The context also notes that the things Paul instituted in that little group of believers were not in huge edifices as seen in America and Europe; they met in homes.