Killing Sin Does Not Consist in the Improvement of One's Natural Temperament

In the fifth chapter of Owen's work On the Mortification of Sin, he discusses what does not pass for truly killing sin. He refers to five things that are not true mortification. I reproduce the third one here because of how often we go wrong here today:

The mortification of sin does not consist in the improvement of a quiet, sedate nature. Some men have an advantage by their natural constitution so far as they are not exposed to such violence of unruly passions and tumultuous affections as many others are. Let now these men cultivate and improve their natural frame and temper by discipline, consideration, and prudence, and they may seem to themselves and others very mortified men, when, perhaps, their hearts are a standing sink of all abominations. Some man is never so much troubled all his life, perhaps, with anger and passion, nor doth trouble others, as another is almost every day; and yet the latter hath done more to the mortification of the sin than the former. Let not such persons try their mortification by such things as their natural temper gives no life or vigour to. Let them bring themselves to self-denial, unbelief, envy, or some such spiritual sin, and they will have a better view of themselves.

 — John Owen, On the Mortification of Sin (vol. 6 in The Works of John Owen; ed. William H. Gould; Carlisle: Banner of Truth, 2000), 25.