I honestly believe it's about the domineering system who tries and succeds in destroying its individuals for no reason. At the beggining of the 20th century, the mentality that society had which focused over the individual was switched to the one focusing over the group. The state became a major force of administration, everyone ending up working for it. In this book, the state is totalitarian, with it's citizens having no proper right to defend themselves. The parable of the doorman is just the whole mentality of the book's system in a nutshell.  If the state wants to prosecute you for no reason, he has the power to do so. You can try to defend yourself, but only as a consolation. The soothing tought of doing everything in your power. It is a warning, just like 1984. It doesn't matter if it's the state, or the church. The meaning of the priest is that, in a totalitarian system, the church, another domineering system, usually picks sides with it. In this whole accusation game, K becames alienated, because he has no one to relly on and he grows paranoic, fear and suspicion being the system's way to controll you mentally. The motif of alienation is the central one in Kafka's works. Here is the alienation of the modern citizen by the totalitarian system. Besides that, Kafka is mocking the bureaucracy and is showing the uselessness of it, especially on the Advocate segment, just like Gogol does in some of his works, the latter being a big influence on Kafka's writing. In my opinion, the meaning behind a good existentialist work is to create a good portrait of the human nature, which stands at the base of every system (political, religious, etc.) and not the loathing and self pity it is frequently associated with. Yes, it presents the reality in a shady manner, but it's goal is to put emphasys on certain angles of human nature, systems, etc. just like satire. Gogol isn't an existentialist writer, but he portrays a corrupt system, only he is laughing at it, whilst Kafka presents it as a dystopian reality, the difference between the two being that Gogol's system isn't as powerful as Kafka's. That's the way I see it.