Main reason why this 2000 year old letter addressing the Corinthian church is so SADLY applicable to the modern church is that at the core the issue is the same: Selfishness.
The main challenge to the Corinthians was to live out what they professed to believe. It is easy to fall into the trap of saying one thing, thinking your life is reflecting what you say, but living out something that is contrary to what Christ wants.
Watch this overview on the book of 1 Corinthians, which breaks down the structure of the book. Paul shows how all of life’s complexities can be solved by replacing self with Christ.