King Silas is King Saul and David Shepard is the shepard boy David that went on to be the King of Israel. The King's son, the black preacher and most of the key players are common to the Bibical version

There is no back story at all. It is just an alternate reality. I thought it would have been better if they had somehow set up how the world has come to that point. A Civil War would have done nicely. The writers even said that they chose not to "waste time" on a back story.

The King has some strong qualities but he shows weakness when his B/inlaw tells him he will ruin him by pulling all the gold out of his banks if he doesn't break the treaty and in the beginning just after saying he would not negotiate with the Gaths (or whatever), he turned pale and was screaming for the Gath ambassador when he learned his son was a hostage.

It is an odd mixture of military tactics with the trenches and modern equipment and it is definitely good TV when you compare it to the usual fare of cop series and reality pap.