"Live by the Spirit, I say, and do not gratify the desires of the flesh. For what the flesh desires is opposed to the Spirit, and what the Spirit desires is opposed to the flesh; for these are opposed to each other, to prevent you from doing what you want. But if you are led by the Spirit, you are not subject to the law. Now the works of the flesh are obvious: fornication, impurity, licentiousness, idolatry, sorcery, enmities, strife, jealousy, anger, quarrels, dissensions, factions, envy, drunkenness, carousing, and things like these. I am warning you, as I warned you before: those who do such things will not inherit the kingdom of God." - Galatians 5:16-21 (NRSV)
We are called through our baptism to 'fight under the banner of Christ' (as Rowan Williams likes saying) against the world, the flesh, and the devil. The apostle actually outlines the 3 ways of living presented to us today.
1. The works of flesh: sin. I like that St. Paul is translated here "to prevent you from doing what you want". He knows the struggle against the flesh and that it is what we want.
2. The Law. God's moral precepts without grace and Christ's merit, without the work of the Spirit, basically to be left alone in Original Sin without the possibility of obedience.
3. The Spirit. The solution of St. Paul is life in the Spirit, against the flesh and the law. The freedom to follow Christ in loving God, and bearing one another's burdens. This the apostle calls "the torah of Christ" (Gal. 6:2)
and if that wasn't enough motivation St. Paul's call for imperfect contrition at the end still rings true:
If you do these, you will not inherit the kingdom.