A powerful examination of both the ways people fail to live up to their uncompromisable ideals and fantasies, but also how attempting to live up to those ideals at all still has merit and carries weight.
The protagonist, the whisky priest, understands that latter point better than most in the novel, and finds it easier to make his peace with the inevitable pains and compromises of life, the beauty in the muck. The lieutenant is less inclined to do so, at least initially, but shares the same great faith in a certain moral narrative (his political rather than religious) and commitment to an ideal seemingly impossible to reach without compromise.
Neither priest nor lieutenant can fully realise their ideals in the world (or state, rather) they live in, and both struggle with how best to embody their shining principles in the living world. Things seem much easier for martyr Juan, a figure quickly removed from the complicated realm of the living and subsumed into a religious narrative where he can embody perhaps unreachable idols.
The stories and ideals characters fill their heads with are in many ways misleading, but they drive them and motivate them to persist through their suffering. Is that noble, or tragic? The novel ends on both notes. I give it four bottles of brandy, and a Bible.