...Sin, as Paul speaks of it, is not first of all a moral category but a religious one. He does not suggest that every pagan and Jew is locked in vice. He would grant - if pushed to it - that both Jews and Greeks could be virtuous. Immorality is a sign and consequence of sin, but it is not sin itself. The opposite of sin is not virtue but faith. Thus, sin and faith are the two fundamental responses of a human being to God: "Whatever does not proceed from faith is sin" (14:23). In such contrasts, Paul speaks of sin in the singular, because it is a rebellion found not in multiple acts of moral failure, but in a basic disposition, or orientation, of human freedom. It is a turning away from God.
At root, sin is the disposition that strives to establish one's own existence and value apart from the claims of the creator God. It is a refusal to acknowledge contingency and dependency on an absolute other; it is idolatry. This disposition is what Paul terms "life according to the flesh," for it measures reality apart from the transcendence of the spirit. He calls it boasting, for it involves a self-aggrandizement that asserts the value of the self at the expense of others. Refusing that side of contingency that is the gift of being from another, idolaters seek to construct life and worth out of their effort, in effect establishing themselves as the god of their own lives. This requires such ceaseless toil and vigilance that, combined with the darkening of the mind that results, it leads to slavery. If the human person is locked in this orientation, then morality, virtue, and even the observance of Torah's commandments can be an expression of sin. They all can articulate the human attempt to establish life and worth on one's own terms. Virtue can therefore be a source of boasting over another person who is immoral. But such judgment is itself a hostile expression of the flesh, and an expression of sin (2:1-3). Likewise, observance of God's commandments can become a form of boasting (2:23), as one attempts to achieve righteousness apart from God's granting of it.
Through all this Paul virtually makes sin into a personified entity, giving it at times an almost mythical coloration (see 5:12-14). This is because he views human freedom as being inevitably in allegiance with, and in service to, some greater spiritual force, either the spiritual systems of idolatry or of the one true God (6:15-23). But the capacity for choice remains as a potential, even when the human being is "enslaved" by the "power of sin." If liberated by the gift of knowledge and love from the Other who was once refused, the human being can be made truly free in faith.
- Luke Timothy Johnson, The Writings of the New Testament, 3rd ed., p. 309