What Is Sustaining Grace?

Here's a really excellent poem by John Piper titled "What Is Sustaining Grace?":

"Not grace to bar what is not bliss,
Nor flight from all distress, but this:
The grace that orders our trouble and pain,
And then, in the darkness, is there to sustain."

I think the hardest line in the poem is "The grace that orders our trouble and pain". It would be easy to think that God is the source of that pain and trouble. But if you read the manuscript for Piper's message (found here) he states that God is actually permitting and measuring out the pain in an exact amount, which is not beyond what we can cope with. He then sustains us through this dark period. His reason for doing so? He works all things for good (Romans 8:28) and we can trust He is working out a greater end and purpose than we can see.