What is doctrine?
Doctrine is what must be held straight in what one believes concerning eternal destiny. It is also what is needed for living the Christian life today. Scripture distinguishes doctrine from learning, which explains why doctrine exists and provides the Old Testament background.
Les explains that Romans 15:4 says things written aforetime were written for our learning. He draws a distinction: doctrine is what we had better have straight in what we believe regarding our eternal destiny, and doctrine is also what we need for living the Christian life today. Learning, by contrast, is to understand why we have this doctrine. The Old Testament remains apropos for our learning, giving the background of how all this fell in place, so that God can now turn to the whole human race with His plan of salvation. It was written for our learning that through patience and comfort of the Scriptures we might have hope.