Of course a quote from ‘Lord of the Rings’ can’t be skipped. There is this moment in ‘The Fellowship of the Ring’ when he tries to block the Balrog. In that moment, Gandalf seems larger than life to me, he appears to grow to the Balrog’s size when he plants his staff in the stone and yells, “You. Shall. Not. Pass!”
A great little tidbit of information is that J.R.R. Tolkien fought in World War I and this is where apparently the quote came from when a French General – Robert Nivelle – said this during the Battle of Verdun (“Ils ne passeront pas!”).