A man may as well conclude that he is a believer because he is an adulterer, because David that was so fell into adultery, as conclude it from the signs foregoing; which are the evils of sin and Satan in the hearts of believers. The seventh chapter of the Romans contains the description of a regenerate man. He that shall consider what is spoken of his dark side, of his unregenerate part, of the indwelling power and violence of sin remaining in him, and, because he finds the like in himself, conclude that he is a regenerate man, will be deceived in his reckoning.
It is all one as if you should argue: A wise man may be sick and wounded, yea, do some things foolishly; therefore, every one who is sick and wounded and does things foolishly is a wise man. Or as if a silly, deformed creature, hearing one speak of a beautiful person, should say that he had a mark or a scar that much disfigured him, should conclude that because he hath himself scars, and moles, and warts, he also is beautiful. If you will have evidences of your being believers, it must be from those things that constitute men believers. He that hath these things in himself may safely conclude, “If I am a believer, I am a most miserable one.”
– John Owen –
from The Mortification of Sin in Believers, volume 6 of Works, pages 49-50