Specifically, the lines are from Book VII, where the poem describes the creation of the universe. They are spoken by the angel Raphael as he recounts the story of creation to Adam.
Here's the full passage:
"His hand hath made this noble work which stands
Glorious Handiwork not by hands;
The world, that vast Theatre of Man,
A stage so proud as this, could not have been
Built, but by Omnipotent Architect:
He laid the groundwork deep, and from beneath
Brought up the heavy Pillars which support
This spacious Firmament; and with vast Chains
Did bind the Globe, and in the lowest Deep
Hung down the vast and ponderous Earth,
And tempered it with Waters."