The story of the two bishops is the famous story that Christopher West often uses in talking about concupiscence. When he talks about it in Theology of the Body Explained, it is part of his commentary on John Paul's talks 38 and 39 (as numbered in the Waldstein translation.)
Before talking about the two bishops, it's important to understand what the Pope is saying in those talks (which appear in the section in TOB entitled “Whoever Looks to Desire…’). This section is part of chapter 2: "Christ Appeals to the Human Heart."
In this chapter, the pope speaks about Jesus' words in the Sermon on the Mount concerning adultery committed in the heart. Talks 38 and 39 are in a sub-section "Commandment and Ethos." There, John Paul compares and contrasts the Old Testament Wisdom tradition on purity of heart with the new ethos of the Gospel. John Paul makes the following important points:
1. Jesus’ words in the Sermon on the Mount are “a direct transition to the new ethos” p. 278 (TOB 38:1)
2. John Paul says that Jesus shifts the meaning of adultery from the body to the heart: “It [this shift] consists of words about desire.”
3. The man who “looks to desire” is the man of concupiscence p. 279 (TOB 38:2) “…because he shares in the concupiscence of the body, he ‘desires’ and ‘looks to desire.’”
4. John Paul says that in the Wisdom tradition, woman “appears more often as an occasion of sin” ie., the woman herself is seen as the occasion of sin. This is important as it will shape societal norms about modesty. (The ultimate expression of seeing woman herself as an occasion of sin is the burqua--not a Catholic concept.)
5. On the text, “Turn away your eyes from a shapely woman” (Sirach 9:8), John Paul comments: “We might say they [the Wisdom texts] develop a specific moral psychology, though without falling into psychologism” (TOB 38:5) This is interesting because on the previous page he had said that in regard to concupiscence, “we are concerned here not only with a psychological interpretation, but at the same time with a theological interpretation” (TOB 38:2); he seems to be saying that the Wisdom texts are concerned with the moral psychology, but we need to go deeper and that is what Jesus does in the Sermon on the Mount.
6. John Paul says that “They [the Wisdom texts] are in some way close to Christ’s appeal to the ‘heart’ reported by Matthew (see 5:27-28) though one cannot say that they show any tendency to transform ethos in a fundamental way. The authors of these books use their knowledge of human interiority to teach morals within the limits of the ethos that prevailed in their historical period and that was substantially confirmed by them” (TOB 38:5).
7. “Such a transformation of ethos had to await the Sermon on the Mount” TOB 38:6
8. In TOB 39 John Paul moves on to examine this more closely. He speaks of "looking with desire"; the desire is still an interior act of the heart expressed in the way of looking at the woman “The look expresses what is in the heart. The look, I would say, expresses man as a whole. If one assumes in general that man ‘acts in conformity with what he is,’ (operari sequitur esse [operation follows being]), in the present case Christ wants to show that man ‘looks’ in conformity with what he is: intueri sequitur esse [looking follows being.] Through the look, man shows himself on the outside and to others; above all he shows what he perceives in his ‘interior.” (TOB 39:4)
9. John Paul says: “Already in the look, ‘in the way one looks,’ it is possible to grasp fully what concupiscence is. Let us try to explain it. [Lustful] ‘desiring,’ ‘looking to desire,’ indicates an experience of the value of the body in which its spousal meaning ceases to be spousal precisely because of concupiscence.” (TOB 39:4)
10. When “Christ says, ‘Whoever looks at a woman to desire her,’ that is, whoever looks with concupiscence, ‘has already committed adultery with her in his heart.’… Does he not mean to say thereby that precisely concupiscence—like adultery—is an inner detachment from the spousal meaning of the body?” (TOB 39:5)
John Paul's thought here could be summed up as: Who we are is how we look.
Looking follows being.
This is the crucial point, that the look, and the way one looks, with concupiscent desire or not, comes from the heart.
The other crucial point is that John Paul is very clear that Jesus is speaking of a new ethos. This new ethos fundamentally transforms the way we need to understand the Wisdom texts of the Old Testament. The new ethos is only made possible through the grace of Christ.
Concupiscent desire violates the spousal meaning of the body.
All of this is important to keep in mind in thinking about and evaluating the story of the two bishops and concupiscence.
But more on that later.