John Owen on the law of sin:
The law of sin always abides in the soul. It is never absent… The law of sin is not simply a visitor, coming at certain times and season. It is at home in the soul.
It is always ready to apply itself to every end and purpose that it serves. “When I would do good,” said the apostle, “evil is present with me” (Romans 7:21). So you never accomplish good– when you pray, when you give alms, when you meditate, or when you do any duty to God with love for Him– without this troublesome, perplexing indweller being there to handicap you. Sin adheres as a depraved principle.
It applies itself to its work with the greatest facility and ease, like “the sin which doth easily beset us” (Heb. 12:1). It has no doors it has to open. It needs no engine by which to work. It lies in the mind and in the understanding. It is found in the will. It is in the inclinations of the affections. It is therefore so easy for it to insinuate evil in all that we do and to hinder all that is good. It has such intimacy to the soul. It possesses all those faculties of the soul whereby we must do what we do. It is always resident in the soul.