Like many of his respected counterparts, Buñuel is someone who liked to include religion in his films. Sometimes in more abstract or philosophical ways, sometimes very directly, as is the case here. It's a topic I'm not particularly interested in, though there's more than just religion to bite into.
Nazarín is a priest who lives in a very poor community. He tries to live his best life, sticking to his beliefs as much as possible, while doing good for the people around him. Those people don't really respect his efforts though, and many of his good deeds seem to backfire on him. With every setback, Nazarín just seems more determined to continue his mission.
So religion, a mix of poverty and misery porn, and a bit of misanthropy. It's a tired cocktail of dramatic elements, not helped by poor performances and bland direction. It might be interesting if you care for Buñuel's worries, ponderings, and/or world view, for me it was just a drag to get through.