What is the theme of "Half-Hanged Mary"?

Margaret Atwood wrote "Half-Hanged Mary" about her famous ancestor who was accused, convicted, and hanged for witchcraft—but miraculously survived and lived for another 14 years. The poem captures the hour-by-hour suffering of Mary and shows her resolve to live. There are several themes that run through the poem, but one is that in spite of impossible circumstances born out of fear and lies, people can persevere.
Atwood uses vivid imagery in the actual hanging to show the merciless way Mary was hanged. She says that due to their haste and poor planning, they used a rope; if they had thought about it further, they would have found an axe. And—

Up I go like a windfall in reversea blackened apple stuck back onto the tree.

Using the Biblical allusion of an apple and then pairing it with the color black, the reader understands that her accusers see her as only a physical representation of sin itself. She follows by noting that the safe place is in the background and in those who are pointing fingers at her. These people are those she has helped in crises of their own. But to help Mary is too risky, and no one dares move from the safety of the circle of accusation.
As she hangs there for hours, she struggles with God, asking if it was her free will to end up hanging from a tree for crimes she didn't commit. But she comes to one resolution in her prayers:

Call it Please. Call it Mercy.Call it Not yet, not yet,As heaven threatens to explode inwards in fire and shredded flesh...

Here we find the core of Mary's desires on the tree. In spite of the betrayal, in spite of the false accusations, in spite of being hung and left to die for horrific crimes she hadn't committed, Mary wants to live. She wants to persevere over her accusers.
And persevere she does. Her spirit does not fail her, and when they cut her down, Mary is alive. In spite of lies born in fear, Mary triumphs over seemingly impossible circumstances and uses that experience to restructure the way people treat her in the years that follow.