Keep...Go Set a Watchman is a bible verse

[GK4Herd]

I'm about half way through Harper Lee's, Go Set a Watchman and come across this quoted passage:

"My text today is taken from tne twenty-first chapter of Isaiah, verse six:

For thus hath the Lord unto me,
Go, set a watchman, let him declare what he Seeth."



Any insight into that verse? I had no clue that title was a bible verse.