The steadfastness of our minds abiding in their duty is the cause of all our unmovableness and fruitfulness in obedience; and so Peter tells us that those who are by any means led away or enticed “fall from their own steadfastness” (2 Pet. 3:17). And the great blame that is laid upon backsliders is that they are not steadfast: “Their heart was not steadfast” (Ps. 78:37). For if the soul be safe, unless the mind be drawn off from its duty, the soundness and steadfastness of the mind is its great preservative.
– John Owen, The Power and Efficacy of Indwelling Sin