On Matthew 5:28:
The Pope [John Paul II] acknowledges that Christ's words about lust are severe. But he asks, are we to fear the severity of these words, or rather have confidence in their power to save us? These words have power to save us because the one who speaks them is the "Lamb of God who takes away the sin of the world" (Jn 1:29). Most people see in Christ's words only a condemnation. Do we forget that Christ came into the world not to condemn but to save (see John 3:17)?
Christ's words about lust call us to "enter our full image". As part of the heritage of original sin, lust obscures in each of us God's original, beautiful plan for sexual love - but it hasn't snuffed it out. The Pope insists that the heritage of our hearts is deeper than lust, and if we're honest with ourselves, we still desire what is deeper. If the human heart is a deep well, it's true that murky waters abound. But if we press through the mud and the mire, at the bottom of the well we don't find grime and sludge. We find a spring that, when activated, gradually fills the well to overflowing with pure, living water. This spring is the "deeper heritage" of our hearts. John Paul II proclaims that the words of Christ reactivate that deeper heritage, giving it real power in our lives.
- Christopher West, "Theology of the Body for Beginners"