494. XIV. THAT ADULTERIES OF THE THIRD AND FOURTH DEGREE, WHETHER COMMITTED IN ACT OR NOT, ARE EVILS OF SIN ACCORDING TO THE MEASURE AND QUALITY OF THE UNDERSTANDING AND WILL WITHIN THEM. That adulteries from reason or understanding, being those of the third degree, and adulteries from the will, being those of the fourth degree, are grievous and so are evils of sin according to the quality of the understanding and the will within them, can be seen from the comments upon them in nos. 490-93. The reason is because man is man from his will and understanding, for from these two exist not only all that is done in the mind but also all that is done in the body. Who does not know that the body does not act of itself, but the will by the body? and that the mouth does not speak of itself but the thought by the mouth? Wherefore, if will were taken away, action would cease in a moment; and if thought were taken away, the speech of the mouth would cease in a moment. Hence it is fully evident that adulteries committed in act are grievous according to the measure and quality of the understanding and the will within them. That they are grievous in like manner if not committed in act is plain from these words of the Lord:
It was said by them of old time, Thou shalt not commit adultery; but I say unto you, That whosoever looketh on the woman of another to lust after her, hath committed adultery with her already in his heart. Matt. 5:27, 28.
To commit adultery in the heart is to commit it in the will.  There are many reasons which cause an adulterer not to be an adulterer in act and yet an adulterer in will and understanding. Men abstain from adulteries as to the act from fear of the civil law and its penalties; from fear of the loss of reputation and hence of honor; from fear of resultant diseases; from fear of upbraidings by the wife at home and thence intranquillity of life; from fear of vengeance by the husband or by relations, and from fear also of a beating by the servants; from poverty or avarice; from weakness arising from disease, abuse, age, or impotence, and so from shame. If from these and similar causes one restrains himself from adulteries in act, and yet favors them in will and understanding, he is still an adulterer; for he nevertheless believes that they are not sins, and in his spirit he makes them not unlawful in the sight of God. Thus he commits them in spirit, though not in the body before the world. Therefore, after death, when he becomes a spirit he speaks openly in favor of them.