Temperament is a poor test of sanctification

…for by what a man is overcome, by this he is enslaved. –2 Peter 2:19

The mortification of sin consists not in the improvement of a quiet, sedate nature. Some men have an advantage by their natural constitution so far as that 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 vigor 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, Of the Mortification of Sin in Believers

Every unclean thought or glance would be adultery if it could

Sin aims always at the utmost; every time it rises up to tempt or entice, might it have its own course, it would go out to the utmost sin in that kind. Every unclean thought or glance would be adultery if it could…every rise of lust, might it have its course, would come to the height of villany: it is like the grave that is never satisfied. And herein lies no small share of the deceitfulness of sin, by which it prevails to the hardening of men, and so to their ruin (Heb. 3:13) — it is modest, as it were, in its first motions and proposals, but having once got footing in the heart by them, it constantly makes good its ground, and presseth on to some farther degrees in the same kind.

–John Owen (1616-1683), Of The Mortification of Sin in Believers