OK, this might seem incredibly simple, but generally speaking, fingers usually belong to somebody. This is our first clue that the speaker thinks there's someone or something out there that makes things happen in the universe.
Of mansions never quite disclosed And never quite concealed (5-6)
This image elegantly summons up the nature of faith in the unseen – it's never fully visible but always somehow apparent.
The Apparatus of the Dark To ignorance revealed. (7-8)
These last lines suggest that <em>some</em> aspects of God are revealed to us. Since we can see God's works (like the wonder of the lightning bolt), the speaker thinks this "Apparatus" – the natural world – is proof that God, or something <em>like</em> God, is out there.