Let us remember that the setting of this incredible novel points towards a number of different viewpoints that are equally acceptable in terms of religion and lifestyle. Antonio is a character whose very life is straddled between different lifestyles and choices. Not only is his house positioned between the plains that his father longs for so deeply and the fertile ground that his mother yearns for, so Antonio is caught between Catholicism and a more animistic, traditional form of religion through the ubiquitous presence of the Catholic church and then the appearance of the golden carp and the magic that surrounds its arrival. The setting therefore points towards a view of the world where magic and mystery is definitely present and an important aspect of life, whilst also indicating the way in which such contrasting views seek to claim humans to believe exclusively in their way of looking at the world.
This novel is above all a protest against adopting any one view of the world as being all-encompassing and providing all the answers we are looking for. This is the critical element that your question points towards. Antonio, as the novel's protagonist, eventually accepts the fact that he does not have to adopt any one of the variety of identities that surround him to the exclusion of all else, but that he is able to incorporate different elements from the rich plethora of cultural and religious identities in which he has grown up. Ultima is the central character that helps Antonio realise that to become a better person he needs to not adopt any one exclusive view but actually build a composite identity. Exclusivism, the novel seems to suggest, will only result in an extremely partial and unhelpful view on life.