It occurs through his coming into touch with the good side of the Force that is within him. His son, Luke, is how this is facilitated. It is salvation through inner means, not from a savior whom God provides. Lucas is trying to find the unifying elements of all faiths to accomplish redemption through human self-will. It cannot be done.
Darkness is bad.
Evil exists both without and within. We need heroes and they can be part of our redemption. But he assumes something that is devastating to his theology. He assumes that we have a measure of good within us an ability to tap into the good side of the Force. We just need the right set of circumstances and challenges to move us to tapping this inner goodness.