The problem of continuing sin in the believer
God's grace is sometimes spoken of as deliverance, when we are freed from the bondage of sin, and sometimes as restitution, when we renounce the old nature and are restored to the image of God. Sometimes it is described as regeneration, when we become new creatures, and sometimes as resurrection, when God causes us to die to self and by his power raises us to life. However, we must understand that deliverance is never so complete that no part of us remains under sin's yoke; that restitution is never such as to efface all traces of the earthly man, or to do wholly away with the old nature. As long as we are confined to this bodily prison, we always carry with us the remnants of our flesh which thus limit our freedom. That is why the believing soul, from regeneration onward, is divided into two continually warring parts. For insofar as it is ruled and governed by God's Spirit, it has a love and yearning for immortality which leads and provokes it to righteousness, purity and holiness. Hence its sole thought is for the blessedness of the heavenly kingdom, and it wholly longs for fellowship with God. But insofar as it retains its natural inclination, it is mired in the earth's slime, entangled in evil desires, and does not know what to aim for or where true happiness lies. It is held captive by sin and is turned away from God and his righteousness.
This produces a conflict which sorely tries the believer throughout his life, because he is raised high by the Spirit but brought low by the flesh.
In the Spirit he yearns fervently for immortality; in the flesh he turns aside into the path of death. In the Spirit he purposes to live uprightly; in the flesh he is goaded to do evil. In the Spirit he is led to God; in the flesh he is beaten back. In the Spirit he despises the world; in the flesh he longs for worldly pleasures. This is no idle speculation divorced from our experience of life; it is a practical doctrine whose truth we experience for ourselves if we are God's children.
So we see that flesh and Spirit are like two combatants laying separate claim to the believing soul, and turning it into a battle-ground. Yet it is the Spirit who wins out in the end. For when it is said that the flesh turns the soul away from God, distances it from immortality, stops it following holiness and righteousness and alienates it from the kingdom of God, we must not think that its temptations are strong enough to overthrow and destroy the Spirit's work and to extinguish his power. God forbid! The truth is that when the flesh strives to pull man down, it burdens the Spirit's work; when it seeks to divert him from his path, it slows and impedes it; when it tries to suppress in him all love of righteousness, it weakens it somewhat; when it contrives to blot it out entirely, it makes it flag a little. In the midst of such difficulties, God's servant must be so roused that his heart's chief wish and inclination is to yearn for God, to endeavour to seek him out and continually to sigh and lament because his flesh does not allow him to press on as he should.
This is the point Paul makes when he says: If we are God's sons, let us not walk according to the flesh, but according to the Spirit (Rom. 8:12-14). When he speaks of conflict he affirms that the Spirit of God is the stronger and that he will win. It is easy, then, to see the difterence between the natural man and the regenerate. The natural man is pricked and goaded by his conscience so that he does not completely slumber in his sins. Nevertheless he is disposed with all his heart to enjoy them, to revel in them and to give them free rein, fearing nothing except the penalty which he knows awaits all sinners. The regenerate man, on the other hand, clings with the chief part of his heart to the righteousness of the law, detesting and loathing the sin which he commits through his weakness. It pains him, he cannot condone it, but instead takes pleasure and delight in God's law and finds it sweeter than all the world's enticements. Moreover he never sins knowingly unless it be against his own inclination, for not only his conscience but part of his feelings are opposed to evil.
–John Calvin
“Institutes of the Christian Religion”