"Throughout his letters, Paul examines [the] conflict between good and evil in the human heart. … For [the Manichaeans], it was a struggle between matter and spirit — the spiritual part of man is good, but the body is evil. For Paul, however, it is not that simple.
"When Paul uses the terms 'flesh' and 'Spirit," he does not mean body and soul. Instead, his use of 'flesh' is synonymous with John's description of lust in its three forms. Flesh … is the part of [man's] heart dominated by sin. A person who lives 'by the flesh' is driven by desire for that which is 'of the world.' The man described by Jesus, who looks upon a woman lustfully, is a clear example.
"The Spirit of God — who lives in the heart of every Christian — desires something entirely different. God wants us to … develop a disposition to purity. This strength is not our own — it is the power of Christ operating with us through the Holy Spirit.
…
"… Life in the flesh began when sin entered the world. Before that time, Adam and Eve lived entirely by the Spirit. With sin came death and decay. But Paul [anticipates] the final victory over sin and death. The first sign of this is Christ's resurrection. 'If the Spirit of him who raised Jesus from the dead is living in you, he who raised Christ from the dead will also give life to your mortal bodies through his Spirit, who lives in you' (Rom. 8:11).
"Our only hope for victory is justification in Christ. This justification is not just a promise of something that will happen in the future. It is a real power operation within the heart and expressed through our actions. …
…
"The acts of the flesh [listed in Galatians 5:19-22] spring from a heart given over to lust. The fruits of the Spirit flow from a heart given over to Christ — that is the way of redemption.
"The fruits of the Spirit aren't acquired without effort. Behind each of these virtues — love, joy, peace, and so on — there are moral choices, acts of the will. But it is only through the power of the Holy Spirit that a person can overcome the flesh and choose goodness. Spirit-driven acts are not human 'works' — they are gifts of God."
In this section, the Pope quotes the passage in Galatians which I cited. Earlier, he also quotes Romans 5:5 and 8:5-11. If you read those passages, it may make this section of the book even clearer. I did not include the passages in order to keep the post from getting too long.