I think it's clear that the "man" in question here is Don Jose, not Decoud. On the surface, this means that Antonia is subjecting Don Jose's body to this trial in the name of Don Jose's spirit, a rather merciless "belief." But one can dig a little deeper here, and suggest that "the man" Antonia loves is mankind itself, when spurred by noble ideals to heights of sacrifice, courage, constancy and suffering (to use her own words ). It is this vision of "Man," struggling to the death against the pain and chaos of life on Earth, in which Antonia, the novel's allegory for idealism, believes.