“The Lord hardened Pharaoh.
Thus he did not recognize him.
The result was
That his works might be known
Under heaven.
His mercy is manifest
To the whole of creation.
He divided
His light
And his darkness
With a plumb line.”
Sirach seems to say that the Lord hardened the heart of Pharaoh in order to show off his majesty. His great power was manifested in all his creation, including his mercy. The Lord has divided the world into light and darkness using a builder’s plumb line that measures things.
Eugene Finnegan Bible Project 