Why does Reverend Sykes make Scout stand up at the end of the trial? And where does Heck Tate get the expression "let the dead bury the dead"?

Heck Tate gets this expression from something Jesus says. 
He tells his disciples that they should not worry about earthly things.  Even if their
father has died, they should let the dead bury the dead and they themselves should worry
about their souls.  This is from Luke, Chapter 9:

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He said to another man, "Follow me."

But the man replied, "Lord, first let me go and bury my
father."

60Jesus said to him, "Let the dead bury their own
dead, but you go and proclaim the kingdom of
God."

As to why Sykes makes
Scout stand up, he does not exactly say, but it appears that he wants Scout to stand up
out of respect for her father.  Sykes, of course, is black and he wants to honor Atticus
for what Atticus has done in defending Tom Robinson.