What did people who only knew the Jesus of the earthly ministry not know about Him?
Those who knew only the Jesus of the earthly ministry did not know the Christ of Calvary, His shed blood, or the power of His resurrection. They missed the work of the cross, which Paul preached as Christ crucified, the power of God to the saved.
Les explains that the people in view loved their favorite song and would sing to guitar music about Jesus walking on the water. That was the only Jesus they knew—the Jesus of earthly ministry. They knew nothing of the Christ of Calvary, nothing of the shed blood, and nothing of the power of His resurrection. Les adds they are not alone, because most of Christendom knows all about Jesus, Christmas, and Easter, yet the power of the work of the cross has been long forgotten. Paul preached Christ crucified, which is the power of God to those saved and foolishness to the lost, and he warned about another Jesus or another gospel.