So, mortal sin is the death of the soul, but venial sin is the disease of the soul. Those who willingly allow themselves to fall into such infirmities and imperfections, which are not yet mortal are like men who are making bad blood, men in whom morbid humors are accumulating a lingering malady upon them. More than this, venial sins have the effect of giving a perverse inclination to the will. The will once united with God, and converted to God, has begun gradually to avert itself from God.
There is no such thing as an equilibrium between God and sin; that cannot be; and when the will loses its union with God, it immediately inclines itself towards sin.
Once more, such sins displease God ; and can any sin be small which displeases God?