What Is Sustaining Grace?

I received a free sermon CD from Desiring God Ministries a few months ago, and on it was John Piper's sermon, Sustained by Sovereign Grace - Forever! I've listened to it three times so far. I would highly recommend it to anyone. Here's my favorite line that repeats again and again throughout the sermon. It is a poem he created to help answer the question, "What is sustaining grace?"

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

Let me know what you think.