I assume you are talking about the story "Unzen" by Shusaku Endo.

Suguro makes the trip to Unzen because he wants to follow in the footsteps of two men.  One is the unnamed man in "Christian Confessions" and the other is Kichijiro.  Both of the historical figures were weak men who were not brave enough to be martyred like the other Christians had been.  Suguro wants to follow in their footsteps because he feels he is just as weak as they were.

Suguro's guilt over being so spiritually weak shapes his responses to what he sees on the tour.  It's sort of like he's doing penance.  Everything he sees makes him think of the martyrs and how much braver and better they were than he is.